At my printing workshop I gave everyone templates for some basic items to print and sew up at home. The sketchy instructions I gave along with the templates need some fleshing out, so here goes:
To make a simple zippered cushion cover, you'll need a front piece and 2 back pieces; the back pieces are half the size of the front piece, plus a little bit extra to accommodate the zip seam. About 1cm should be sufficient.
Zigzag stitch around all the pieces, then attach the zip. Right side of zip to right side of one back piece, stitch along the red dotted line.... then flip it over and attach the other edge of the zip to the other back piece (right sides together).
Lay the front piece down right side up, open the complete back piece out flat, wrong side up, and lay it on top of the front piece. Open the zipper a few centimetres, and pin the whole lot together.
Stitch along the dotted red lines. Slow down as you get to the zip, and stitch really really slowly right over it. Trim the ends of the zip, and turn right side out.
An envelope cushion cover is even simpler.
Again, you'll need a front piece and 2 back pieces. This time, add a bit more length to the back pieces before cutting out: half the length of the front piece, plus 2cm for hem allowance, plus 5 to 10cm for the envelope overlap.
Zigzag around all the pieces. Then hem the back pieces: fold down 1cm of fabric to the wrong side, fold it down again, and stitch along the red dotted line. (Ironing the creases before sewing makes things a bit easier.)
Lay the front piece down right side up, lay the back pieces down right sides down and overlapping, matching the edges carefully, and pin the whole lot together. Stitch along the dotted red lines, then turn right side out.