First, remove the old wrap & guide. Using a razor blade, cut from the end of the guide foot towards the center of the guide. Go slowly & make many passes if you don't cut all the way through the thread the first time. Take great care not to cut the rod shaft. Now peel away the old wrap & the guide will be free from the rod once you've cut the thread from both guide feet.
Next, clean up the rod shaft where the old wrap & guide were with wet 1,000 grit sandpaper. Gently sand away any ridges of old varnish, making the rod at this spot smooth. Be careful not to sand too much. Don't remove any of the rod's varnish on the shaft or the color coating of a synthetic rod - just the old wrap varnish.
Now take the new replacement guide & grind down the feet of it so that there is a gentle, curved shape to each guide foot. This is easily done with either a fine cut metal file or quickly by just kissing the guide foot on a belt sander. This will make it easier to wrap the thread over the guide foot where it meets the rod.
Now take the thread for the wraps & put it under slight tension. There's many ways to do this. You can make your own tension device (like the one pictured below), buy a commercial rod wrapper, or simply put the thread spool in a bowl & run the thread through the pages of a closed book. Either way you'll need some kind of slight tension on the thread.
Continue wrapping the the thread from the spool onto the rod, making sure each pass of thread lies next to the previous one with no gaps in between the thread. Every now & then you'll have to push the turns of thread together to avoid gaps. You can use either your fingernail, a smooth bone-folder, or a similar tool like the round body of a writing pen. Push the thread together gently - you don't want them to roll over each other - you want them to lay together with no gaps in between.
As you continue to wrap up & over the guide foot try to keep an eye on the tension of the thread. When you get to a point where you're about six rod revolutions or so from the center end of the guide foot, stop. Take a loop of thread made from another spool of thread & lay it under your last wrap. Make sure the loop is pointed in the direction you're wrapping (towards the center of the guide). Now continue to wrap over this loop for the last few wraps.
-Practice on a wooden dowel rod before you try this on a rod.
-Matching the color of the old wraps: thread color will change when coated with epoxy or varnish. Try a few different spools of thread that are close in color on a dowel rod. Coat them with your finish to see which one matches your old wraps.
-Especially in the beginning it's much easier if you have something to hold or cradle the rod for you. You can use a sturdy cardboard box with one side cut away & V grooves cut into the sides. Be creative.
-There are many different ways to wrap a guide. No one way is better than another. Eventually you won't need any tape to hold the guides down or anything to cradle the rod for you. All the techniques I described are for someone trying this for their first time. Good luck!!!