chair cushions

[satabor]

Recently, I acquired 2 darling upholstered chairs from Habitat for Humanity.  The chairs are old, but unused (had been stored in a warehouse), so only the foam seat cushions need to be replaced.   I purchased a slab foam 5' X 24' X 72' and each cushion  is 19 x 19 (zippered).    Can I get step by step instructions so that it can be done properly?  Thanks and bright blessings!