Ken,
The trick here is to make sure that:
1) you only have one copy of each solid
2) Only select two solids at one time to difference
3) select the solid you want to keep first and the "cutter" second.
So here we go. If you have a large square solid and your extruded surfaces (letters) then you would make sure nothing is selected to start with. Next click on the operations button, and then the difference button. Then move you curser around until the large square turns
RED , and left click.
If you will notice, before you left click, at the bottom of the screen in the info box it says "select the object you want to modify". That's the large square.
Then it will say"select the object you want to remove". This will be the individual extruded letters. So select one of them. One is probably already
RED , but you can select any of them. Then left click and it will remove it from the large square. Leaving a gapping hole. JK
Before going to the next one, do a render to see your handy work. Here is were you puff your chest out and think "man am I good" do a little dance, call everybody in to see how smart you are, ect...
Then come back to earth and remove the rest of the letters.
And please change your sig as no one here is an Idiot.