I posted your pic for you here hope you don't mind.
This is quite the challenge especially with tshirt fabric. But i really like your idea.
I would cut out a template out of cardboard for each of the peices so you can lay them on the tshirts to cut out.
I think your easiest way to do this is to cut each shirt up the sides and under the arms into two pieces the frint and back.
I would sew the peices together using red thread on to of the reds and yellow on the yellow. I think it might be easies this way. I would get yellow and red ribbon and stitch them onto the points where the tie is to be. then you can tie them together or another neat look might be to sew up the sides again once all pieces are together then let the ribbons hang loose from the side seams.
I hope I'm explaining this alright for you.
Since sewing on tshirt fabric can be tricky I'd use a narrow zigzag stitch. on it. This would also give some extra embelishment to the whole project.
I think you might want to practice one with a couple of old shirts that don't matter first to be sure it's going to work out the way you planned.
good luck and be sure to post a pic when they are done!