Here’s how to make the applique:

Start with these basic shapes cut out of some felt:

Pin the big hearts onto the onesie with the ends meeting and, with a sewing machine (or by hand), sew them on near the raw edge of each heart with pink thread.

Stitch the small heart onto the body piece just before adding the body and the head on over the wing hearts with red thread.

Then stitch on the heart tips for the antennae, hand embroider the eyes with french knots and hand stitch the other details with backstitches in red floss.

I put the onesie, tutu & hat on my little girl and presto!  She’s turned into a Love Bug: