WobiSobi: No Sew, Tee-Shirt Halter #3, DIY. This site has a ton of different ways to cut up an old ugly tshirt to something cute!
How-to: Cutting the Perfect Off-the-Shoulder Tee | Boundless Beauty
how to cut a t-shirt the easy version - The title doesn't lie, it doesn't get much easier than this. Makes a blah t-shirt much more summery and free flowing.
Fitted Tee Tutorial - Scoop Neck | a.steed's.life (actually there is no sewing involved with this one!) Just snip with scissors, but she shows you how to do it correctly.