There are a million videos on youtube that shows the trick. Simply go to the browser (with the memory card icon and where you'll see a disc if you put a valid disc in) and put in an original ps1 game. Once you see the disc loaded on the browser, just swap out using a swap trick (I can't remember if the slim has the flip top or not, if it doesn't you should get one), push x to load it, and it'll load.
You need to block certain sensors to do any sort of swap trick.