The first thing to be aware of is that the color of the thread will change under the finish you use. It doesn't matter if you're using silk or nylon thread; real varnish or epoxy finish; the color you see on the spool will change when you hit it with your coating. You can use a product called "color preserver" to keep the color you see on the spool intact. It has it's drawback & benefits. They also make some nylon threads with color preserver already infused in the thread. When these first came out they weren't that great, but I think they're getting better. Either way, you'll want to test the color under your finish on a test wrap on a wooden dowel rod. I've talked about this before, with more detail & you can read that HERE.
The next thing you'll need to know is how to wrap the guide onto the rod. There are several ways to do this. It's not very hard to do, but it does take a little practice. I have a detailed article all about this, aimed at helping the beginner, over on the Tackle Tips page.
So now you've got the guide wrapped & secured onto the rod & you're ready to coat the entire wrap with some form of finish. If you're only replacing one guide, than match the finish with what's already on the rod. In other words, if the rod's wraps have been coated with epoxy than make sure to use an epoxy rod finish. Hint: all professionally made bamboo rod wraps are coated in wood varnish & most (but not all) of the mass-produced graphite rods today have their wraps coated in epoxy. You can tell at a glance because epoxy is a lot thicker & heavier than wraps covered in varnish.
You'll need something to cradle the rod while you work & while it dries. The rod needs to be horizontal for both. It can be something as simple as a long cardboard box with 2 notches cut out at either end, but you do need something to hold the rod.
It's always best to put lite coats of finish on at a time. It's always better to apply 4 or 5 lite coats than 2 or 3 heavy ones. You want to build up the finish over several applications. Apply one coat a day & let that coat cure for 24 hours. If you apply too heavy, or thick, of a coat there's a good chance that it won't cure correctly & that it will sag & droop to one side of the wrap. If that happens you'll have to remove the wrap & start all over again to get the best results.
After you've applied each coat, you need to turn the rod so the finish doesn't droop & sag to one side of the wrap. If you have a drying motor, great - use it. If not, then simply turn the rod a half turn every 10 minutes or so for, say, 2 hours or until you're sure the surface of the finish is set up & it won't droop.
As you go along, adding coats of finish, you'll probably have some bumps from the tag ends of the thread & some bubbles or other imperfections, etc. That's okay. The wraps won't be pretty until the end. They're going to look something like this:
Now the next time you have to replace a guide &/or wrap you'll know that it isn't that complicated. Go ahead & give it a shot. With a little time, patience, & practice you'll be finishing those wraps like a pro!!!