Gluing and stitching is best, but if you want to do it yourself, I am posting this because I have “walked out” of a lot of felt soles I replaced until I took the time and patience to go through the following steps……………….
Finish taking the loose sole off. Or, if the soles are simply worn but still attached, try the following:
Make sure all areas are TOTALLY DRY.
Rough up all glue surfaces slightly with the largest grit garnet paper you can find (40-50 grit). I usually mount a small hand held belt sander upside down in a large bench vise and use it, but the heavy garnet paper should work if you apply a lot of time and effort.
Cut the soles to size out of the felt you have, but cut so that the new soles are about 1/8 to 1/4 inch less in width all the way around. Reason: the edges of your old soles will be rounded all the way around the boot sole. If you cut your new felts to that exact same footprint, you will be attempting to glue and hammer your new felt to those rounded edges. Your attempts will tend to resist and pull away from that rounded edge, so cut the new felts slightly smaller.
On first coat of contact cement, IGNORE the directions on the can !
Apply one coat of WATERPROOF CONTACT CEMENT to all surfaces that need to be glued (DUCO is a good brand but any at the hardware store that say WATERPROOF).
Wait approx 24 hours.
This first coat will form a base that will absorb into the felt and make a surface for the second coat to stick to. (If you depend on one coat alone, the felts will later fall off for certain.)
Then go back and do a second coat and follow directions on the can. They may say something like allow to dry until tacky, like 20 minutes.
When you stick the felt back on, make sure you have it in the right place the first time.
Now, put a scrap block of 2×4 up in the toe and / or heel.
Go to a stair step and put the toe area on the step with the felt facing up.
Beat the felt as if you mean to kill it, all over repeatedly with a hammer, so that the felt will stick for good. The scrap block of wood should be on the back side to provide a substantial solid support as you strike w/ the hammer.
If it is the heel, stick a scrap length of 2×4 up against the inside of the heel area and do the same thing with the hammer.
If you follow these directions, it should stick for good (most of the time).
Take my word, my first wife’s father was a shoe repairman and I worked in his shop some. Plus, I have walked out of a LOT of newly installed felt soles that I put on before I learned these tricks. Most of my education has been the result of making more mistakes than most people are allowed.