i would wait til your son is in bed, and lay the material out on the floor.

decide on the width you want, and fold material lengthwise on this line. measure often to make sure it is the same width all along. you could iron or pin the material along fold. cut along fold. (*make sure you include the amount you need for hems as LS described)

once you have it cut, fold over hem 1cm and iron. then fold over again 1cm and iron - this way you won't see cut edge. To do this with long material, i would have it looped over ironing table kinda like this (___/--\____), pulling material towards you as you iron the hem and then have it layered near your feet. The prep will take a little time, but it will help to get the width all even when you sew.

when sewing, i would set the machine up at end of a kitchen table, and have material stretched out on the table. you want to make sure it can go through the machine without it being pulled back by anything.

The side that is not cut, you don't have to hem OR you could fold and hem just 1cm instead of 1cm twice.

Because the wrap will only by 70cm wide (or whatever you decide), you should be able to lay it flat on the table while you are hemming the edge.

just take it slow and you will be fine. and always measure twice and cut once!