Here is what you will need to applique a t-shirt:
-2 copies of your motif (with pieces numbered if it's complicated)
-fabric scraps for the motif
-Heat N Bond Lite iron on adhesive
-fiberglass ironing sheet (or you can get away with an ordinary press cloth too)
-Sulky Tear-Easy stabilizer
-thread- shiny embroidery thread if desired
-Pellon Easy Knit fusible interfacing
I recommend pinning a copy of your motif to your garment first to double check it for placement and scale. Try it on. Look in the mirror. I was way off on my original idea for the scale and wound up increasing it quite a bit to look intentionally oversized.
Heat N Bond Lite to the wrong side of your fabric scraps. The "lite" version is plenty strong enough for the t-shirt and most other things. Peel off the backing. Using one of your copies as a pattern, cut your motif out of your adhesive-backed fabric scraps. I decided to do a batch of shirts at once, so layered up two at a time to save on some cutting time.
Sulky Tear-Easy. It is lightweight and very tearable. It gave enough support just pinned on at 4 corners behind my motif.
Now you're ready to do the applique stitching. There are a lot of different machine stitches that will work. I like a blanket stitch because it looks hand-done. I like zig-zag too. Satin stitch (zig-zag with a short stitch length) looks like what a factory would do. And there's always plain straight stitching if you want is to look clean & simple. There are many choices of thread too- shiny, regular, variegated. Always experiment on a scrap first. Pivoting around curves an corners is tricky (only pivot with needle down), so practicing first is key. While you're stitching, be careful not to catch more than just the outside layer of the shirt in the stitching. Machines with a free arm are great for this, but I don't have that, so I have to be careful and smush my fabric around just so.
Pellon Easy-Knit interfacing with the grain matched up to the grain of the shirt. Trim it to shape first so there is not too much excess around the edges, but the whole back side of the motif is covered. This will protect the stitches that you so painstakingly knotted off and make it nice and smooth next to the skin, especially for kids.
If this tutorial helped you, please send me a picture of your results!