Your picture frame can be any size you want, mine is 283mm x 283mm. The aperture is 207mm x 207. I happened to have a lovely decoupaged frame created for me by my sister, but it can be any frame. You can get some lovely frames in charity shops.
You will need:
Remove the glass and backboard from the frame.
Then cut the fabric 125mm wider than the frame backboard. Place the frame facedown and then place the fabric rightsize down into the aperture in the centre of the fabric.
Place the stuffing onto the fabric aperture until you are happy with the result. Keep checking the plumpness of the aperture area to make sure it has enough stuffing to make a good pin cushion
Replace the backboard and pull the fabric until it is tight
Fix the backboard securely into the frame using the metal tabs and cut the corners of your fabric to reduce the bulk. Glue down the fabric using the glue gun to finish off and neaten the back of your pincushion.
Next, cut a piece of greyboard to the same size as the backboard. Then cut a piece of fabric that will be big enough to cover your greyboard and wrap over onto the back to secure. Glue the excess fabric onto the back of the greyboard with the glue gun.
Place the covered greyboard wrong side down (so the lovely fabric covered board faces upwards) onto the back of your pincushion and glue into place.
Using PVA glue stick the embellishment onto the corner of the pincushion. I found PVA better for this as the glue from the glue gun worked initially by broke off when knocked. I can only thing that the shiny surface on my frame accounted for this