I recently took a t-shirt that I liked, but was way too big and remade it to fit better. It was a simple project and thought you might be interested in the process.
I initially used one of my fav long sleeve t-shirt and laid it out on my cutting table, Then put a piece of pattern paper on top and traced the shapes....sleeve, back, front and then added 1/4" seam allowances. You could add 1/2" seam allowance if you would prefer.
I folded the sleeve pattern in half and laid it on my shirt and cut it out. I also saved the hems on the sleeves....I didn't bother to open them up.
I did take a few minutes and a seam ripper and took off the neck ribbing, mainly because I wanted to keep the width of the ribbing, so I could reattach it later.
Lay the front pattern on the front of the t-shirt after cutting the shirt up one of the side seams so you can lay the shirt out flat to work on it. And cut the front out.
Then repeat for the back of the shirt...laying the pattern out and cutting around it.
I next stitched up one shoulder seam using 1/4" and then added the neck ribbing back on stretching it slightly as I stitched. Then stitch the other shoulder seam....stitching through the ribbing too. Press the seam allowance away from the ribbing. Top stitch on the right side through the ribbing with a larger stitch length.
Then stitch both side seams and under arm seams all at one time. Notice I was too lazy to take out the hem of the shirt....I left that in as well and just stitched through it and back tacked to secure the seam.
Turn right side out and it is done and fits me wonderfully!!!!