I’ve never replaced gear cables before - always left that to the mechanic 
But as I’ve started servicing more of my own bike I’m wondering if I could do it. I have a Giant Revolt Advanced, so internal routing but not through the headset. Shimano GRX groupset.
Any advice on dos and don’ts? I’m particularly interested in tips or tricks you’ve learnt that make the job easier 
Any preferred YouTube instructional videos? How long should I expect this to take?
Much appreciated 
Its just straight up easy. Having internal routing doesnt matter because the cable housing is already run through the frame. Just remove the nut on the derailleur, expose the shifter, and push it back up through the housing. Then send the new cable down and pinch it off on the nut. Make sure to unscrew all barrel adjusters if you have any.
Make sure you don’t mess up that dab of solder at the end of the cable, or you’ll have a problem pushing it.
I haven’t had any issues with internal routing the couple times I’ve had to do it. I try to go slow, but keep it moving, and don’t try to force it if it hesitates…just back up and try it again, maybe spinning the cable (spin it the non-fraying direction).
It should be pretty easy. My go to channels for this stuff are Park Tool and Free to Cycle 
Replacing the cable is fairly easy. Just make sure to release the tension first (using the shifter put the chain in the smallest gear). That will align the holes in the shifter.
The hard part is getting the cable tension correct. That’s best explained via a video but I’d advise starting by turning the barrel adjusters in and then back out one or two turns.
If the housing is old replacing the cable may not help any shifting issues. If you slide off the end caps from the housing and see a couple mm of the individual wires, the housing should be replaced.
And I always replaced the housing anyway. Some housing wasn’t lined, and that that was lined sometimes developed holes and slices in the liner from the cable moving inside. And don’t try to yank the cable when tightening the bolt as it’ll be too short and you’ll have to redo it. The adjusters should be about 1/3rd to half of the adjuster showing so when the cable stretches the slack can be taken up without loosening and tightening the hold down bolt. (It can do damage to the cable and potentially cause it to splay or break individual wires. (and ALL cables stretch eventually, even the ‘pre-stretched’ ones))
Great tip!
I have a Revolt w/non-headset routing so happy to be of help here. If you’re able to take the crank and bottom bracket out, it will make things go much faster, you have a much larger buffer for error.
I think anything, the first time, takes a lot longer than you expect.
Right, it’s the housing the wears. If the cable didn’t get kinked or develop broken strands, it’s fine (but replace it anyway). The plastic lining in the housing can wear through and start to create friction, or wear products from it can gum up the movement. That’s a common problem with teflon coated cables where the teflon can slough off. Cables with hard coatings, typical black or greenish in the case of Shimano Optislick cables don’t have that problem. My current preference is for polished Sram inner cables and Shimano SP41 housing. Existing internally routed cables/housing/hoses can almost always be used to guide through the new ones as the new ones are pushed through.
Exactly this about the first time taking a long time. It keeps getting easier after that. I bought a cheap internal routing kit on AliExpress and it’s been super useful.
Bike shop employee cheat here: I love using cable liner temporarily to chase cables with bikes with internal routing of cable only to maintain the old line if the bike isn’t “tubed through the frame”. Basically snip the old cables where they enter and exit through the frame (don’t lose them in the frame/use tape to keep them from pulling through), run liner, run new cable, then remove liner. Sometimes bikes with internal frame routing will have the housing stop at the frame and run bare cable, then restart at the exit. I did an older Revolt a while back and it was like this. Tons of carbon Specialized are like this. I think I had to pull out the head tube built in cable stop and then reinsert it after I threaded cable because it’s too small for liner but at that point the hole is large enough to grab it. The chainstay was the worst, it pulled out before I ran liner and had to use magnets to fish it through. I always replace housing when I do cable if it’s been over a year (and then also bar tape because it’s off already) the lever feel on new cable and housing bikes is sooo amazing–you don’t realize how good it can be since it slowly got so bad. Especially true if the bike came with the coated cables as mentioned above, the coating just sits in the housing and can cause friction. Some Shimano shifters slowly will break cable threads at the head. If you are doing all that work, do it right. Whatever you do, don’t yank all the cable and housing out first without a way to chase the line through the frame, will be way less headache with a plan.
That’s a great recommendation 
. Didn’t even realise cables were made differently (seems obvious though )
That is an awesome cheat!
What if you wanted to replace the housing and the cable at the same time? Asking for a friend who just purchased an XTR shifter cable/housing set and has a hardtail with internal cable routing? Should I try to do it by hand or just bite the bullet and get a internal cable routing kit?
Use the Seldinger technique:
What I’ve done is using a ling cable, threaded the cable through the new housing, threaded the new cable through the old housing, and then just pull the whole thing through. The cable will hold them together and if they get stuck, you can usually pull back and forth to get it through. Also keeping the housing taut in the frame sometimes causes it to hang up, just keep the cable taut in the two housings until the old one comes out. Then PULL THE CABLE, and clip the housing to fit, reinsert the cable and finish up. I’ve used a similar method with hydraulic hose: Put a cable in the hose and pull it out, then run the new hose up the cable until it’s out. It’s a little finicky at times because the cable can bend, but it saves having to buy a hose routing tool. I’ve tried taping the hose together but some frames have tighter tolerances and things the tape can get scuffed on going through. 
Similar to the method just above, but using the existing housing helps the new one pass through easier. The shop I worked at had tandem cables and they worked brilliantly for this as they were really long…
And a gripe: The exposed wire style on so many bikes was a PITA because it provided so many additional areas where dirt and water could enter the housing and even the liner. It makes it easier to stretch the cable after installation, but the added ingress points can insure you’ll be doing that again soon if you use cheap parts. Get housing with o-ring end caps and hope they fit in the brazons that hold the housing. Many of them will not accommodate them, sadly…
For internal routing, I push the new housing or hose through with it linked to the existing housing/hose to guide the new one. I use 2.5-3’ length of brake cable (slightly fatter than shifter cable) to link them. As mentioned, if it hangs up, a little back/forth/twisting will usually free it. You might try leaving a cable, pulling the old housing out, and then try to push a new housing over the cable. This can work, but stands a much higher chance of hanging up on something and not being able to get past it.
Depending n how the home is set up (only upper and lower housing and exposed cable in the frame vs housing through the frame), id just marry up the old cable (and or housing) with the new one with some electrical tape and pull the old one down from the rear derailleur to “fish” it through the frame for you.
Or just have a shop do it if that sounds too hard or finicky. But that should work fine so long as you secure the new and the old together well.
Edit: Noticing now that I’ve repeated what the other two have said, but yeah ditto to them and that’s what your shop would do for you. If there is no housing and your going blind, you can use a coat hanger or cable like to hook to grab the wire (totally sucks)