This is the last onesie that was shipped up to New York for my BFFs daughter a few weeks ago. You can check out the other ones here and here.
I just did a simple, standard, applique process. I use fusible interfacing on all my appliques. In the past I did not use it, then I did a project with letters, that tend to be more noticeable if they are crooked, so I tried it. Ever since I used it that once, I was hooked! It is not that expensive (especially on sale!) and it really gives a nicer finished product.
I googled clover/shamrock to find a shape that I liked. I cut out a square of fabric big enough, and ironed on fusible interfacing on the wrong side. Then I traced it onto the paper side of my fabric using my laptop screen as a light-box.
I cut out my shape. Ironed it on to the cotton onesie. Then I carefully sewed around my shape using a zigzag stitch.
I thought I was finished but I thought it didn't look girly enough for that cute girlie so I made a little bow out of a green polka dot fabric and hand sewed it onto the stem of the shamrock. Perfect!