mending can be different things. Here, I have "let out" a shirt by creating a gusset in the seams. This is a pic of the finished work before pressing .
First step rip out the old seams.
Next cut fabric for the gussets, I separate the sleeve from the torso.
Then, straight stitch, and overlock stitch.
Next step trim extra fabric, get ready to hem and finish
Don't forget to press it!