There is one forbidden character in the line of definitions of new-rules, which is the "," character which is used to separate rules.
 

Every rule begins with the character "_", so that any character to the left of this character is ignored. In particular if you make many rules (like me) this is useful for writing comments of what each rule does.
 

Go into your configuration and look for the following configuration option:

read its help. It works in the same way as

and you need to have the last one configured to save to some folder. Do not use this configuration option for saving messages from an incoming folder collections. It was not designed to be that way and will lead you to errors. There are some ways to solve this problem by adding a new function to the rules patch that I talk about below, but I do not think this is necessary for the mean time.

Now below this options you will find a new configuration option, which are the rules (or hooks). They are very simple, you should have a line that looks like

Now what you are going to add in there is the action that has to be done (saving to a specific folder). There are many actions that can be taken, because new-rules can be used to do many things, but this is the way it should work for you.

Assume that you were subscribed to the Lynx developers list, whose subject always contains the string "lynx-dev". Assume that you wanted to save all these messages to the folder "save-lynx", then you would add the following text to the new-rules configuration option.

new-rules has a help text that explains what all the tokens and separators mean and how to use them.