1. Decide how many beds to devote to the crop.  I keep notes on how much I preserve of each type of vegetable and of the month when I ran out of those stored foods in the winter.  If I had to buy tomatoes starting in February, that's a clue that I should plant more beds next year.  On the other hand, if I ended up with peppers that I didn't want to eat when the time came to clean out the freezer in the spring, I might as well grow fewer this year.  Don't get too carried away, though ---  if this is your first or second year gardening, you'll want to keep your garden small and manageable.

2. Consider where the crop will grow best.  I like to save the sunniest spots for crops planted in the early spring or those which will survive late into the winter.  The next sunniest spots go to tomatoes and cucurbits that succumb to fungal diseases during our hot, humid summers.  Herbs can go anywhere, but you'll use more if they're close to your front door.  Root crops require deep, well-drained soil, so keep them out of clayey or swampy spots.  If you hand water, you might want to keep moisture lovers like celery close to the hose.

3. Hunt and check until you find a spot.  Now that I know I need 10 potato beds and that the root crop needs to be located in the loamy third of my garden, I can start hunting through the the garden until I find the appropriate number of beds that haven't grown tomatoes, potatoes, peppers, or eggplants for a few years.  With a spreadsheet, you can simply search for the bed number (or sort by bed if you have all of the information on the same sheet) and get a list of all of the vegetables grown in each bed since you started taking notes.  Although a bit time-consuming, this hunt and check method only takes me a couple of hours when deciding on spots for all of the crops in our huge spring and early summer garden.