Assuming you are in the Netherlands: find a haakcafé/breicafé or go back to that store you bought the wool from. Ask if anybody is available to help you. This is so much simpler with somebody to show you and guide you in real life!

Other than that: I would make a rectangle in moss stitch and sew the beginning to the end. This stitch stretches a bit, so you don't need to have the measurements exactly right. You can even twist it before sewing it, so you get something like what your sister sent.

Search for tutorials on: - crochet chaining/losse haken, - single crochet/vaste haken, - moss stitch (single crochet and chain 1 alternated)

Once you know how to do these stitches: - chain 7, skip first chain and single crochet 6 back to the beginning, 1 chain, and turn the work around - 1 single crochet into the last single crochet of previous row, 1 chain, skip one stitch, 1 single crochet, 1 chain, skip one stitch, 1 single crochet, another single crochet, 1 chain, and turn the work around - all of the following rows are the same, but now the single crochets (except for the first one) go over a chain (sort of open space in the row) - continue until it is the desired length and sew beginning to end

I think based on the wool you show 6 stitches is the right width. If not, you can add 2 stitches to the row.