If you use the "duckbill" type magazines, then no conversion is necessary. You may need to trim the wood around the mag well for it to fit in, but other than that it's a "plug and play" type deal. Popping out the trigger group and removing the original magazine is something you should be able to do anyway, to clean the cosmoline out of the guts of the rifle.
If you're wanting to convert it to take AK magazines, that's a different story. It will involve some machine work.
Be advised, however, that taking an imported SKS and replacing the original fixed magazine with a detachable one is a violation of Federal law. To do it legally, you need to replace enough parts on your SKS to make it legally a "US made" rifle. It's a convoluted thing, and I'll go into detail if you want, but for now I'll stop by simply saying that using detachable hi-cap mags on an otherwise standard SKS is a violation of Title 18 USC, Chapter 44, Section 922(r). It is a felony, and carries a hefty penalty if you ever did have the misfortune to be prosecuted under that charge. This is one of the things that most people don't understand, nor do they want to understand, about the laws concerning SKS rifles. They think that just because hi-cap detachables are commonly found and cheap, then they must be legal to install. They think that just because the Assault Weapons Ban died last year, it must be OK to put whatever you want on an SKS. That is not the case.
If you want some light reading, a friend of mine has put together an "SKS FAQ" page that goes into detail on all the legalities concerning the SKS rifle.