measure the item you want to insert diagonally from one corner to another, say six inches. Then cut a square (always a square even if the insert is rectangular) of paper one inch bigger ie 7 inches square.
place the insert on the paper and fold each corner of the paper loosely over the insert towards the centre, crease the folds and then put the insert to one side
where the four creases intersect at the edges, cut out the triangle overlaps as shown
stick double sided tape along both sides of one corner (in this case the westerly corner with my mug on it) and stick the adjacent flaps to it to form an open envelope
round the shoulders and tip of the remaining corner for neatness and seal as you choose (stickers can be usefully used up here)