A club sandwich is just a double decker sandwich, a sandwich with two layers: the bottom layer is a BLT (see above) and the top layer is a chicken sandwich (see also above). Some people use just chicken on its own, but I find that the chicken on its own can be very dry. So, I like to use chicken salad in my club sandwiches.
Start by making a BLT
On top of your BLT, spread butter or margarine on the top toast
Put chicken or chicken salad on the top toast of your BLT
Do a 3rd toast, spread it with butter or margarine and mayonnaise and place it on the top. If you used chicken salad, you do not need to put more mayonnaise.
The next part is a little tricky: place your large toothpicks so that they will hold the sandwich together when you cut it diagonally.