I glanced at that webpage, but I'm not sure if it answered your question or not, so here goes:

Canceling a move in this case means you perform one move, and then after it hits, you cancel the rest of move (the recovery animation) and start a new move. The general rule is you can cancel normals into specials or supers, and cancel specials into supers. You can't cancel into ultras.

As for how it's done, the trick is that you need to cancel very quickly, or the cancel won't work. For this reason, the important thing to know is that you can start inputting the stick/d-pad motions for a move before you intend to do that move. So for your El Fuertue example, you can do a think of the double-quarter-circle-forward motion for the super as d,df,f,d,df,f. Then, what you actually input is:

d,df,f,d+K,df,f+K

The first K exectues the crouching kick, the second K cancels it into the super. The whole thing is done as one quick motion. (Alternately you could do d+K,df,f,d,df,f+K; you just need to do the motion faster this way. This wasn't as viable in Super Street Fighter II Turbo, but the timing windows are much more lenient in Street Fighter IV.)

Also, you can't cancel all normal moves. The third series of trials is meant to teach you which normals can be cancelled.