What you’ll need to do is find something with a nice horizontal surface (maybe the bottom of a big box? I use my coffee table)
First, take the bottom edge of the shirt and put it underneath of the box and make sure its remotely even.
Second, your gonna want to set on the box and start pulling up on the shirt. The weight of your body will help keep the bottom of the shirt down.
When your finished the shirt will be well stretched, but not super wrinkled.