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So I got these jeans from my friend in a size 13. I'm a size 7 so I had to make the jeans fit me. They already fit my legs luckily. All I needed was for them to hug my waist.
All I did was take the sides of my jeans and pinch them. I sewed up the pinched parts of the jeans to reduce fabric. You may also cut off the pinched fabric after you sewed it.
Next, after I fitted the pants, I put them on and put a chalk mark where the jeans are at my knees. I cut at that line to make shorts. I cuffed the jeans and sewed the cuff to make it neat and sturdy.
Afterwards I took the legs I cut off and cut each in half
Then afterwards I sewed them together. Short side to short side. So now it's a long piece of fabric. Then I fit the fabric across the my chest and around my back to make a tube top. (Now, I'm pretty flat chested and if you aren't, I suggest getting few sizes up in jeans since you can adjust them to fit you later, you can have access fabric for your top when it comes to your bust. For reference, mine was a size 13.