If you have the old tiptop measure the ring outer diameter and order the same size. If you don't have it, measure the outer diameter (in millimeters, or convert to mm) of the topmost guide and order that or something close to it. Size is not that critical to function, but if you go far off the right size it will look goofy.
The tube size is measured in 64ths of an inch or millimeters. Measure the diameter of the clean blank at the tip and order the next size above this diameter. You can order the exact size, but might get into a "too tight" situation, and better to get it a little larger and just use generous adhesive.
The first number in a tiptop size designation is the ring diameter in mm, the second is the tube diameter in mm or 64ths of an inch. They come in different finishes, try to get the finish that best matches the guides.
Most builders like to use hot melt designed for tiptops as it makes future replacement very easy, but some prefer an epoxy. Epoxy makes the next change, at times, very difficult. You don't need to use much heat. Best way to take the old one off is to apply a little heat from the side of a butane lighter flame while applying tension (string, rubber band) to the tiptop and it will come off when the adhesive is ready. If it doesn't come off with just a little heat, stop and get help from a pro builder. It is easy to ruin a blank with too much heat. I usually can remove and install while handling the tiptops with my bare fingers. That's how little heat it takes when you know what you're doing.
To install the new, cut little chips off the hot melt stick and insert them into the tube of the tiptop. When you get a generous amount in there, apply heat to melt it and put the tiptop onto the blank, rotating it to the proper orientation before the hot melt solidifies. I should have mentioned before that you should "dry fit" the tiptop to the blank before using adhesive. This is to make sure it will fit properly when you do add the adhesive. Do not sand a blank tip to make a snug tube fit. Open the tube with an awl/ice pick kind of tool, or order the next size up if that doesn't work.