I finished up decorating some plain white onesies we bought for Dalton - had to hurry and do it before he grew out of them! I got the idea for the argyle one from MADE. The other ones use clipart, or pictures from freeapplique.com.
For the appliques, I traced the pattern on the paper side of some Wonder Under, which you buy in the interfacing section, FYI. Then I cut it out, ironed it onto the wrong side of the fabric, then cut that out. Then I peeled off the paper and ironed it onto the onesie. Then I zigzag-ed around the edge of the designs.
For the argyle onesie, I cut diamonds from felt and stitched them on with a straight stitch. Then I embroidered the other diamonds. I should have drawn lines with a fabric pen first, because it's a little crooked.