First of all, you need to sign up for a new account at Betfair.com. For that purpose, you will have to fill up the registration form and then provide your credit card information, which is mandatory to place bets on Betfair.
Your credit card information will be used to draw any amount from your account, if you lose any bet, or to transfer money to your account if you manage to earn some money by winning the bets.
After you are done with setting up a new account at Betfair, you can visit the Horse Race Laying section and place your bet by laying your desired minimum amount at 1.01 odds. For instance, you can place a lay bet for £8.00 at 1.01 odds.
Go to the “My Bets” section of your account and look for the bet you placed. Click on it and modify your bet by increasing the minimum amount by the amount you wish to bet. For example, let's say you want to bet 1p. Just add 1p to your original bet. Now your bet will be £8.01. This will split your bet into two parts, one for £8 and one for 1p.
You can now cancel the £8 bet. Adjust the odds of the 1p bet to what you desire.