Download and print the templates from sewmag.co.uk. Cut up the side edges of the jumper and straight across, just under the base of the armholes. Position the base template, with the straight edge of the template running along the rib edge of the jumper. Leave a 1cm seam allowance at the outside edges of the template, but not across the straight edges of the envelope opening. Snip out two pieces. Using the template, cut out two envelope top parts, marrying the straight edge of the template to the ribbed edge of the jumper.

Separate the pieces into two sets. With one set, lay the top part of the cover over the base part, crossing the ribbed areas over one another so that the top parts of the ribs are laying one on top of the other. Stitch them together across the top of the rib.

To check that the cover will fit the hot water bottle, cross the two pieces of the template one over the other along the dotted line. The top of the rib and the length of the rib will differ depending on what jumper is used. This segment can be carefully positioned onto the cut pieces to adjust the lap over of the ribs. The rib at the top of the base piece can be trimmed back to 2cm. This piece will then become the back of the cover.

For the remaining set of pieces, fold the rib at the top of the base piece inside. Lay the top part over the base with the top of the rib running in line with the folded rib of the base piece. Pin the two layers together at the side edges, then edgestitch the top to the base at the overlapping area at the side edges. Check the overlaps with the template. This piece becomes the front of the cover.

Cut a strip from the jumper, 4cm x 12cm. Fold the strip in half lengthways and stitch the two long edges together, leaving the ends open. Turn the tube to the right side. Fold the tube in half to make a loop, then position it at the top of the back piece, with the loop hanging down towards the base of the piece. Edgestitch in place.

Lay the front on top of the back, with the right sides of the pieces facing each other and the edges matching. Stitch around the edges leaving a 1cm seam allowance, sandwiching the loop at the top into the seam. Turn the cover to the right side through the opening at the front of the cover. Tease out the seam edges and press flat.

Cover the button at the front of the cover and cut a disc from the jumper. Draw around the button onto the wrong side, then snip the disc out with an extra 5mm around the line. Do a running stitch around the edge of the disc. Place the button inside and pull up the thread around the button to encase it. Fasten off the thread and sew the button to the front rib edge of the cover.