Do you have Palmer's book, How to Brew? It is on the web somewhere if you don't have a hard copy. He has a pretty good procedure.
You measure the gravity of the wort after mashing, compare it to the sugar you theoretically could have extracted and divide them to get a ratio you convert into a percentge. Different malts have different potential yields and Palmer's book is the best place I know of to get the numbers you need.
On a brew day, from your process, you need to know the lbs of grain in your mash, volume of water you add to your mash and the volume of wort you extract, plus get a gravity reading before boiling, to calculate your efficiency.