There are several ways of making playtest cards:

Altering an existing card: PREFERRED!

The second method is to use a card that shares some of the characteristics with the card that you want to make as a Playtest Card. Remove any characteristics that do not match, replacing them with the correct characteristics. Methods for doing this can be found on youtube.

Printed Slips:

Print out all 75 cards, and slip the cuts sheets of paper into each sleeve. If you do this, every card must be printed on the same type of paper and have a basic land backing.

Collector's Edition cards:

CE/ICE versions of cards are allowed.

World Championship cards:

Blank WC cards can be used to make great playtest cards with your own writing or art. Make sure that the unique backs of these cards cannot be seen through the sleeves.

Using Basic Land:

The first and most basic is taking a basic land card and writing the details of the card on it. Make your cards as legible as possible - a dark Sharpie on Plains works well. Your opponent should be able to distinguish your cards.

If you have complicated cards that don't have current Oracle text on them (playtest, foreign, altered, or just very old), you might want to print out the Oracle text ahead of time to make it easier for your opponents.