Get a screwdriver. Or a butter knife. Whatever.
Pry the keys off.
Depending on your exact model, this may be all you need to do (...if the switch is recessed inside a guard and the key has a blunt spike that pushes down into it). Or, if there's a switch exposed, you may want to put a rigid thimble over the top of it and stick it in place with blu-tac or similar.
Alternatively look for classic AT keyboards on ebay and invest in one of those and a couple of converters (AT to PS2 and PS2 to USB... make sure the latter is an "active" one because those that are just pinout converters will only work with keyboards engineered to work as AT/PS2 or USB ... someone somewhere may make a straight AT-to-USB converter but I doubt it).