How can you make a Magic the gathering card?

Magic Set Editor: see link

GiMP: see link (although you will need to download templates to use

Adobe CS (much the same as GiMP, except Adobe is not a free download)

Alternativley you can buy a load of commons or land and use an ink eraser or just stick different pictues to the front of the cards. If you have the cards in sleeves the will be easier to use.

NB: It should be noted that proxy cards are not allowed to be used in DCI sanctioned tournaments. There are some tournaments where proxies are allowed. Always check with the tournament co-ordinator or tournament judges before presuming proxies are allowed.