I have altered tshirts for years and years!
You can alter a t-shirt neckline with a easy little 'x' stitch. Have your needle (a chenille is good), Aurifil 12wt or another thread of similar weight. These photos were done a LONG time ago when I did this project with a similar thread.
BUY 12 wt AURIFIL .. the kits are a great way to pick up several colors
You are folding the neckline in half then making 'x' stitches.
Pretty much all it is! For the collar, fold the ribbing to meet the seam.
Start at the side. Hide the knot in the fold.
Stitch 'x's.... until done!
I try and keep the thread inside the ribbing so it doesn't show too much on the inside.
But don't stress over it. The collar is harder to needle than the rest.
I usually run out of the thread because a normal working strand won't last to do all the way around
so I do the front first as it's what I can see in the mirror!
For sleeves and hem. You will fold up towards the FRONT, twice. The first time is where you hide the knot. Start at the side.
Make 'x's... until done!
something I do ALL the time..let me know if you try it!