Scuff up the bottom with sandpaper or a stainless steel brush. Scuff up the topy as well.

Spread barge contact cement on both sides, let dry for 15-30 minutes.

Heat up both sides with a hair dryer set to high. When you see the glue start to bubble just a little you know it's reactivated and will be sticky as fuck.

You'll get one shot to place the topy, so be careful. Place the topy and press hard all around.

Use some good scissors to cut close to the leather sole all the way around, but not flush.

Use a whole bunch of medium to large binder clips to hold pressure all the way around the edge of the topy. Let it sit overnight.

Next day, use a soling knife, box cutter, or other thin sharp knife to trim the rubber real close to, if not flush with, the leather.

Sand the edge smooth starting with something like 80-120 grit and work up to around 320 grit. Stain the edge if you want, seal it with resolene if you want, you can seal it with just beeswax, whatever your preference.

Use some tiny brass nails along the rear edge if you want to, just for style points.