I'd use the "pinch and pin" method, might need someone to help you tho.
Put the tshirt on inside out and just pinch the fabric along the seam so it's a bit tighter then pin it as you go, take it off carefully OR draw a chalk line where the pins are (on both sides of the tshirt) and take them out again. Then just sew the one seam starting at the armhole, going under the arm then down your side as far as you like (or need to). If you're tshirts are the same size you could use this one as a pattern to resize the other ones too. I learnt this method from Threadbanger, I can get you a video if you like, just let me know