Trading method is the way how the script decides which trade it sends the surfer to. It creates a list of all trades, sorts it by the criterions of the trading method and sends the hit to the first trade on the list that wasn't visited by that surfer yet.
To create a new method, type in the name of the new method and a pattern which you wish the trades to be sorted by. Next to the form, there is a list of keys you can use. You can also use parentheses (), substraction -, addition +, multiplication * and division /. This generates number n for each trade. After you finish typing, select whether you want the trades to be sorted ascending (lowest n on the top) or descending (highest n on the top). If you want to see an example, take a look at the default methods.