Find a t-shirt that is VERY big on you. I used my grandpa's old t-shirt, and my grandpa is at least twice my size. I'd recommend one that goes halfway down your thighs.
Turn the shirt inside out. Measure the bottom of the shirt. My shirt was about 24 inches across. So I decided I wanted a 4in space in between the two legs, so that meant each leg would be 10in across. Get it? 24-4=20/2=10. But this depends on how wide your shirt is.
Measure 10 in across and make a mark with the marker or chalk. Then measure 4 inches from the 10in mark. Check to see if the other side is 10in.
Make a 75 degree angle from the 10 inch mark. I did about 5 inches up and it was a little tight in the crotch, so you should probably go about 3 inches. Do the same on the other mark. Do not make the lines meet. Draw a straight line from each line to join them
Here's the hard part. I used a sewing machine, so I'll give the directions for that. I wouldn't recommend hand sewing, just because the seam isn't going to look great. But if that's all you have good luck. So anyway, sew down the first line. Sew down the straight line, and then the last one. (I re threaded my machine each time to avoid bunching).
Cut all the lose strings.
Cut the inside of your stiches. It should look like a skinnier version of this
Turn the shirt inside out.
Now you have to decide how you want to be able to get into it. I cut a wide collar on mine. But you could cut a straight line down the back and make a button hole, and then sew on a button. If you know how to sew on a zipper, you could do that as well. Velcro is another option.
Once you've decided on #9, try it on ! And now you have a new and fun romper :)