Material and tools: cotton fabric 150×50 for the outside of the bag, and another cotton fabric of the same size as lining. Preshrink the fabric before sewing (iron with steam iron or soak in warm water), 1cm thick foam, 4cm thick and 2m long strap, 2cm wide Velcro (wider is better), cotton thread, sling clips, chalk, scissors, pins, needle, stitching thread, iron, ruler, sewing machine.
Place the lining and the outer fabric right sides together with foam on the bottom and pin together. Sew the two shorter opposite sides on the machine, and cut the round edges and corners. Cut out the foam from the seam. (On the picture only the flap is stitched, not all the way to the corner; it would be good to sew the whole side with the corner, then the edge wouldn’t have to be folded). After the fabric is sew to the foam, select a longer stitch.
Pin Velcro to the lining flap, about 1.5cm from the edge. Fold the bag in the place of the bottom, put the laptop into the bag and fold the flap, so you can see where to sew the other side of the Velcro. Sew the flap and front part of the bag 1 cm from the edge, sew around the Velcro 2x.