To get a webcert, you go to the mypoints.com website and click on "spend points." Then, look under almost any category and you'll see (at the end,usually) a link for a Webcertificate. Click on it and then follow the questions/links to cash in the points for the webcert (you can get $10 or $25 or more, I think).
The webcert info will come in your email, sometimes quickly and sometimes more than a month later (depends on where they are in the cycle, which is monthly, I believe). Once you get the certificate, you'll go to Webcertificate's site and set up an "account" (which works like a Mastercard debit account). You'll get a card number (looks like a debit or credit card number) which you can use to purchase on-line items (I don't know if Disney sells AP's via on-line or not).
For MUCH more info than this, go to the Disney Rewards board and do a search for webcertficates. Lots of help there!
Took