I bought a mens shirt at the second hand store for 1€ - sometimes it occurs silly to me how one can spend so much money on a womens shirt from the shops when you can get it so much cheaper.
Get a mens shirt and lay it flat on the floor. That takes a little patience, but it's possible.
Take a shirt that fits you wll and lay it onto the shirt
Copy the shape of the top on the shirt and remove it.
Think of the seam allowance!
Cut the shape out.
Try the shirt on and decide how tight you want your sleeves to be.
Now there are two options:
A. you will be perfectly satisfied with the fit of your shirt
B. there are still some loose spots and parts that don't fit well. Just try it out. But, basically everything is done so far.
Oh, and I also cut the length.