Go to Forest of Illusion 1 as Cape Mario and get past the middle gate. Go to a place where there are many wrigglers and koopas, by the key and lock. Now, jump from badguy to badguy, holding Y and B to make you float slower and have more control. Try not to jump on shells or non-shelled koopas because that can stop your flight. Now, as you bounce back and forth, you will slowly rack up points and eventually begin to get extra lives. When you are getting lives from the koopas, try jumping onto a yellow wriggler. The point value you get will be made of all these weird symbols and will give you tons of points. There are many weird combos of symbols possible.
A Youtube video of this glitch can be seen here.