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I just read your comments and will try to explain as best I can your questions.

I used a pair of underwear I had cut open at the seams so I could see the basic pattern of. That wasn't very clear so that might have been what was confusing. I placed the top of the back of the underwear along the hem line of the t-shirt and layed out my under flat on the t-shirt.
At this point you will probably scratch your head wondering how you'll ever have room for the rest of your bathing suit but you should be ok. I cut the outline of my underwear on the front piece of my t-shirt out and I cut straight up from where my underwear left off to above and around the collar of my t-shirt.

I cut the collar out of my t-shirt and stretched it for my head. I then made slits down to where the front of my underwear started and I stretched my t-shirt out thus making it longer.

I hope that answers your questions. Feel free to ask me more if you need any additional help.

Thanks for favoring my Milla Jovovich T-Shirt Bathing Suit