I wouldn't recommend doing this for a botfarm, however with the free-access weekend happening it's pretty useful..
Step 1. Go to mail.google.com and sign up for a GMail address.
Step 2. Go to Runescape create account
Step 3. Enter in EMail and password and create account.
Step 4. Log out of new account and create another one, this time add +1 to your GMail.
So if you used IBOTALLDAY@GMAIL.COM for the first
You wil use IBOTALLDAY+1@GMAIL.COM
Continue on with +2 +3 +4.
Jagex does limit the number of accounts you can create per IP, so a VPN or a router reset will fix this.