Well I haven't done this specifically, but I believe this is the basic process. Look and make sure the trunion block and stuff above the mag opening is the proper wide standard doublestack piece. Assuming it is you can just cut open the mag opening in the receiver to the proper size for a standard doublestack mag (I'd get a template somewhere online and trace it on or use a mag for a template, cut it undersize and go slow checking fitting until the mag has the desired fit). All you should need to cut is the relatively thin stamped receiver obviously. I would use a dremel, but I guess you could use some good files over a lot more time.
This site has some general AK receiver templates, should be able to use their drawings/measurements of the mag opening to get it done.
http://pookieweb.dyndns.org:61129/AK/docs/templates.htm
Some of the earlier WASRs had narrow single stack trunions I believe, in which case the conversion gets much harder as you need to mill out a good amount of metal from the thick trunion. You'd need some kind of milling junk for that which I doubt you have.
Oh and for legality you need to check the parts count. You want 10 or less foreign parts to be legal. Some singlestack WASRs already had US fire control parts, (should count as 3 parts if so), don't know if they had anything else. If yours is US it should have some markings to indicate so, but I don't recall how exactly to identify them, do a search. You probably need 5 parts total, or six if it has a muzzle compensator, usually everyone does FCG (3), US made pistol grip, and US made gas piston. There's of course the rest of the stock assembly you could replace with US parts (handguards count as 1 part together), etc. Or less desireably IMO you could use some US made mags/mag parts.