Purchase/find/build your daybed platform. For mine, I built it out of a sheet of plywood. For the legs, I attached industrial casters to ready-made legs (which you can buy at most hardware stores). The casters gave the daybed a bit more height, but they also make it really easy to move the daybed around when we're entertaining. I cut the plywood so that the finished size of the daybed is 3 feet by 6 feet, and we have it up against our storage shed so that there's a "back" to it.
Measure the daybed and determine how many pillows you'll need. I recommend two-four layers of pillows (the one in the photo has six layers and that's too many for comfort). So, if your pillows are 36" square floor pillows, and your daybed is 3 feet by 6 feet, you'll need two pillows/layer for a total of 4-8 pillows.
If the pillows aren't very deep (like those in the photo), stitch them together, end to end for a long strip of pillows. This will keep them from moving around. If they are deep (as with floor pillows), attach no-sew Velcro to each pillow to keep them all together.
Pile the pillows on your daybed platform and dive in and enjoy! You'll find this is the favorite spot in the garden for all of your guests, and probably for your pets, too ;-).