First you have to decide which part of your body you'd like to emphasize, (remember: rule of 1). She focused on the waist, with a hint of torso. Where you decide to make the slits will generally determine the silhouette you're creating: vertical slits on the torso will elongate it, horizontal slits on the waist will create an hour- glass shaped outline, and slits on your sleeves can make your arms look skinnier.
Getting them even can get tricky, so make sure to mark your lines before cutting. If the fabric is dark you can always use white eyeliner, works perfectly.
There you have it... a 5 minute long DIY that created such an adorable outfit. Pair your T- shirt dress with knee high socks/ tights and you're golden.