Find a warm, sunny spot to farm freckles in. This could be a park bench, the beach, or even a lawn chair in your backyard. All you need is the sun and a little time (not TOO much time, though - the point is to avoid burns)!
Slather sunscreen over every exposed part of your body that you're not trying to get freckles on. Put sunscreen on the areas where you want freckles, too, just put it on thinly and don't re-apply unless you feel yourself starting to burn. (See warning #1)
Recognize that there's a difference between some mild freckle-getting and burning the crap out of your skin! Use common sense. If you're in a really hot spot, in an area without a lot of ozone cover, or just super-pale, it's wise to heed the old Australian advice: "From 11-3, stay under a tree!" You'll still develop freckles later in the day, it'll just take a couple of days for them to really come in.
Stay outside for about an hour or so, then stop your freckle-farming for the day, especially if you haven't spent much time outdoors recently. Long-lasting freckles develop over the course of several days...if you spend too long outside one day, you risk burns and more serious skin damage. The look you're after is a light sprinkling of freckles, not the leather handbag look!
Enjoy your newfound freckles, and take care of your skin in-between farmings with moisturizers and mild soap.
