I use the OTB Abyss boots that have drain holes on the bottom of the boot. they only have 4 holes that are about a 1/4 inch hole, although they do have a mesh netting to not allow stuff to come into the boot.
"Rivers know this: there is no hurry. We shall get there someday.” ~A.A. Milne
I have done this with old leather boots. It may be a bit more difficult with modern boots with lots of layers of fabric. I used a grommet kit and placed the grommet in the instep close to the level of the insole. I added a layer of stainless steel screen from a faucet aerator (pipe shops also sell fine screen) to the outside before I set the grommet. The grommet kit includes a punch to cut the right sized hole. With fabric layers you may need to melt the edges so they don't come apart. I don't know about that, I'm an old fashioned leather boot guy.
The hardest part of the process was getting the grommet set "anvil" properly supported inside the boot so you can hammer the thing together. I used scrap wood and lots of experimenting before I managed to lined up and supported.
Old boots? Why not? It will probably work out fine and dandy. I like the advice given so far. Probably better than my method which would of been to toss them into the backyard and let my American Bulldog "have her way" with them for an hour......Guaranteed to drain ;-)