For this easy project you will need thread, measure, scissors, pen, needles, shawl and a sewing maschine.
Start with laying the shawl folded on the middle, flat on the floor or a table. Measure from the middle 1/4 of the total waist perimeter to both sides. Measure from the top corners 1/2 of the total armhole perimeter.Mark the sideseam and cut. (I descided to pin the shawl with some needles to make sure the fabric did not move when I cut it.)
Take the loose piece of fabric and place it on the other side. This way both sideseams will look the same. You can also fold the shawl twice, if the fabric is not too thick, and only cut once.
Do also cut a hole for your head;)
Zigzag all the edges and sew together the side seams.
Fold approximately 1cm and sew the zigzaged edge to the wrong side.
Last you cut the fringes (if you have any) as short as you want. You may also want to iron the finnished t-shirt. I'll probably match it with a pair of denim shorts and wear to the beach. If the sun ever comes out...