Why create a custom set of playing cards? Not because you want "vanity cards", there are web sites that will do that for you (another instructable) but because you want a set of cards you can not get, like for a game you are designing or a commercial game that is out of print.
The example I am using in this instructable are cards from the game I am designing called "I, Detective".
Step 1: Gather Your Supplies
1) Your cards
2) Playing cards
3) Cards Sleeves (also known as deck protectors)
I suggest you buy the Cards at the Dollar Store ($1 for two decks at my local Dollar Tree), and buy the card sleeves wherever you can get them cheapest. I have found them online, at big box stores (Target, WalMart, Toys R Us) and game stores for under $5 and sometimes under $2. Typically these come 50 in a pack, so if your deck needs more than 50 cards, buy multiple packs.
Making the cards is the next step, but you should know in your head how many cards will be in your deck.
Step 2: Build Your Cards
Once you create your deck in nanDeck you can generate it as a PDF (for easy reuse or sharing) and print it as well. I suggest sizing the cards so that you fit nine (9) on a page. This makes cards a little smaller than playing cards, but this is fine as you will see in the next step.
Step 3: Generate and Print Your Cards
Step 4: Assemble the Deck
All done! Have fun with your new deck. Also (as I am sure you figured out) you can easily redo the deck at any time. This is VERY helpful when prototyping a new game.