Materials: crewneck sweatshirt or t-shirt
Towel, Polar fleece, or other safe comfort layer (optional)
Thread - does not need to match

Decide if you are using any design that is on the shirt. Remove loose appliques and anything else that might be harmful if chewed and eaten. Pets can't read, but most humans can, so determine if the shirt design is suitable for children and modest adults.**

Turn shirt inside out and sew sleeves closed to create roughly square or rectangular sides. (Rest of instructions will assume you are leaving the sleeves as padding. If you cut off the sleeves, use a low bulk seam finish to prevent fraying or raveling.) Turn right side out and continue. OR pull sleeves inside shirt and secure the openings from the outside.

If retaining one or both sleeves as padding, remove bulk from hems or cuffs. Overlap the sleeves using natural inclination of fabric. Some sleeves will cross each other at the elbow, some will completely over lap at the neckline. Smooth the fabric to reduce large lumps and gradations in thickness. Add comfort material on top of any sleeve fabric to even out material and pad as needed.

It doesn't need to be glass smooth or pancake flat. My own dog just pawed a throw rug into a lumpy mess for her third morning nap. A difference in thickness will allow the pet to control its body temperature a bit as well.

Roll or pin as for quilting and begin securing layers with small to medium stitch length freehand or pattern quilting. Secure sleeve outlines and padding if possible or quilt through and next to additional padding. A jean notion may help with changes in bulk from thin to thick or just raise your presser foot and make it happen. This is a work of love not a work of art.

Sew hems or bottom edges together. On many sweatshirts, the stretchy bottom treatment (cuff material) can remain. Sew near the attaching seam on both sides (in knitted fleece fabric and cuff material) and close to the edge.

Zigzag crew neck and any tags. Fasten off all stitching with backstitches and tie or apply seam sealer (Fray check). Check carefully for any remaining pins.

** To make an R-rated t-shirt into a G-rated Snuggle, use the basic steps above but fold the shirt in half with the design hidden on the inside. A folded XL shirt fits nicely around a hand towel (I SURVIVED DFTM).