You child is outgrowing the high chair and wants to sit at the table with you. Either you use some cushions to prop the child up, which can be dangerous, or you can get a booster seat to help the child sit up in the chair without having to peer up to it. Better yet, build one yourself, it is easy to do, and it will give you the feeling of accomplishment.
Take the two smaller pieces of wood and lay them parallel to each other. Then take the two longer pieces and put them parallel but joining to smaller pieces. You will end up with a square, and then you just nail it together. Remember, you can always adjust the size of the pieces depending upon the size of you chair.
Put the square that you have built down flat in front of you and then put the 2 foam pieces on top of the square. The two foam pieces go on top of each other so the foam is 8 inches high. It will shrink when you put the vinyl over it.
Now take the vinyl piece and lay it right in the middle on top of the foam. Fold one side over the foam past the wood and then under it. Mark it with the pencil. Turn the wood over and then the mark you have on the vinyl will be reduced by about ½ inch so the vinyl will be shorter on the booster chair.
Nail the vinyl in on that one side. The nails go on the bottom so no nails are present to hurt your child later. Then take the vinyl and nail it on the other side at the bottom. The idea is to have the vinyl fit tightly over the foam so there is no sagging.
Turn the seat over and make sure it is all even. You should have the vinyl straight. Now the third side is a little tricky. Pull it down towards the bottom but you will have a flap sticking out on the corners. Tuck the flip in on both sides equally under the vinyl and glue the ends, so the vinyl will not come out.
Now you will nail the third side of the vinyl to the bottom and then repeat the procedure for the fourth side. When you are finished with it, get your child to come and try the new booster seat.