I've picked up a surplus used aircrew Beaufort Mk 10 Immersion Suit (for aviation use, but I think there's more likely knowledge among watersports enthusiasts). It needs latex socks adding to the bottom of the legs.
There are plenty of YouTube tutorials, showing the basic principles which seems to be fairly straightforward - stick the latex sock on the inside of the suit, cover the joint in and out with tape.
Except, my suit has two layers of cloth at the end of the feet. They seem to have old glue inside, so I presume I should sandwich the latex feet between them when I glue?