Lay a piece of paper on top of your tank top and sketch a collar. I added 1/2 inch seam allowance but I wish I had made my collar a lot bigger.
cut 4 collar pieces out of white fabric.
Sew two pieces together leaving a small hole along the top edge to turn it right side out. Snip up to the sewing line and then turn the piece right side out and iron. Repeat with the other two pieces
Position your collar pieces on the shirt and sew along the top edge.
Sew the ribbon down the middle of the shirt (I stitched a line on both edges of the ribbon).
Set in your sleeves.
So this part is a tiny bit tricky you can fit the sleeve to the tank and sew the sleeve closed but know that it will stretch so I’d make the sleeve circle a few inches bigger than the tank armhole. I did not do this so my tank had extra space at the arm pit. I just folded it and called it a cooling area.
If I were to do it again I would stay stitch the tank armhole first allowing it to stretch a little since I don’t like cramped armholes. Then I’d fit the sleeve to the armhole and then you won’t have to worry about the sizes not matching and set in the sleeve how you would on any garment.