The easiest method I know is to :

Compare to a bookies price. Any bookie will do. If you look at the market at least 10 mins before it races then set a price in there at least 25% less then the bookie price.

With 1 min to go check what bets have been taken. Lay them off at the higher prices.

Move to next market.

Its an easy method that works for a few bets each day but watch out for data charges and limits on requests for bets matched, etc.

For example if a horse is at $20 with a bookie then you lay it $15.00. You will likely find betfair has prices like $10 on the back side and $30 on the lay side. Your just looking to pick up the cheap lay and then trade it out.