honestly i just kind of made it up
but i measured around my boobs and just made a long tube like that. i added a zipper and all. then i cut out a small piece of fabric and made a little tube. and then i wrapped it around where i wanted it to be. it slides bc i didnt sew it down.
then for the bottom i just measured it to where i wanted it to go down i just pleated the front a lot. and then i just sewed it all together.\
then i sewed straps to make it a halter. im sorry im no help.
that probably makes no sense at all.
im sure theres tutorials somewhere that have little bits and pieces of what i did.