Underlaying is fairly simple once you get your head around it.
The principle is to reduce your lay risk by a small amount ( say a few pounds). Since your lay risk is what you might win/lose at betfair-what is the purpose? Win less but benefit by busting out of the bookie. Lose LESS if the bookie bet wins, which means you now have more profit which helps when you're completing the wagering requirement of deposit bonuses.
You did the right thing by betting the full £60 ( your 40 deposit and 20 bonus) in one go.
Can you post the odds you were looking at ?