These are both pretty much the same idea as far as making them.

Squares:
1 - Draw a square however large you want this to be, let's say 6 x 6 pixels and fill it with black.
2 - select the black and shrink by 1 pixel
3 - hit delete
4 - shrink by 1 pixel
5 - bucket fill with black
6 - deselect all and you pretty much have it.

The circle is the same thing just use the circle tool.

edit: You would probably want to autocrop and save your final as a brush so you can use them forever without having to recreate them from scratch each time.