Easiest way to prototype a card game?

I use Adobe Illustrator and get 9 cards to a standard card stock sheet (8.5"x11") that I buy at Staples. But I still need to cut the cards. I not too long bought a 5-in-1 paper cutter which helps really nice when cutting and cornering of cards also. It's not perfect - but MUCH, MUCH better than using scissors with paper! I've already cut like over 600 cards... (8th Prototype)

http://www.staples.ca/en/Staples-12-inch-5-in-1-Paper-Trimmer/product_70...

(Sorry it's a Canadian link ... Sometime's it doesn't cut 100% straight but it suits my needs)

You can also buy BLANK white cards from The Game Crafter. They are Poker size and have rounded corners. I have been thinking about buying some of those for my next game (Prototype)...

https://www.thegamecrafter.com/parts/blank-poker-cards

100-999 cards is $0.04 each... That is pretty cheap. As they say great for "Prototyping"...

P.S.: If you are going to go down the home print & cut route, I really would suggest you buy a paper cutter (if you plan on cutting many cards)!

Any software solution is usually OK but it is not as fast as Write & Play for early prototypes. As @corsaire sais, both sleeving paper cards (with a standard poker card as back, for thickness and uniform back) and index cards are great. After some prototypes with both steps (1st index cards, 2nd paper print) I decided to create a basic template for 2.5" x 3.5" blank cards in a 3 x 3 grid in a standard DIN-A4 for home printing. You can print a lot of these or just photocopy one to have a stack of simple blank cards for prototyping.

I recommend having a simple template that you can cut with cutter or scissors. As you were asking for "the easiest" I think this is a good compromise between easy and practical, because index cards can be pretty difficult to shuffle and tend to get scratches and folds. With blank paper cards in sleeves yo get the best of both solutions: durability and easy handling of sleeved cards and fast and clean creation/modifications of index cards. This, also, allows you to think about the future design of the cards when prototyping, because index cards don't have same size or aspect of poker sized cards.

Avery has some pretty decent templates for business card sized cards if you don't mind using smaller cards.

I'd also suggest typing up a few basic templates in Word or what have you. Print those out and sleeve them with card sleeves that have either a standard deck of cards in them, or old magic cards, or what have you.

Sleeving them with other cards keeps them anonymous when face down, and gives them the feel of real cards.

So far I have been using index cards cut in half and card sleeves. Two of the little prototypes I quickly made are just pencil writings on index cards. One (the one I am working on with my 7 yr old daughter) we just used images on the web she liked to print out on cardstock and sleeved them.