Secondhand bookshop to the rescue! I found a daggy but excellent "how to sew" book that gives very, very clear instructions. If you are going for a retro look it also can give you great fashion advice.
To test out the pattern I successfully made cushions for my restored rocking chair.
Today I made the cushion for my husband's study chair from a tapestry his Mum stitched for him.
Firstly I lined the back of the tapestry to protect the stitching.
Then the fiddly job of sewing the top and bottom to the gusset. The key is pivoting the corners.