Where can I modify the overall game command 'sit'?

There are objects in my game you can sit on, e.g' sit on bench, but if player simply types 'sit' it returns "no time for lounging about now", which is fine for rooms that have no objects you can sit on, but not for the others that do.

Thanks.

What exactly should the player type to sit down?

I want the player to just use the word sit, and if there is something in the room that can be sat on, they sit on it. I tried to make the command 'sit' call the object attribute 'sit on' for the object (armchair), but returns error.

Pixie talks about sit in his guide on using verbs:

http://docs.textadventures.co.uk/quest/guides/using_verbs.html

I think you were on the right lines with the custom command, but maybe your syntax was incorrect.

I know how to create a response for a custom command, but unfortunately not how to assign it to an object.

If there's only one place to sit in each of the rooms in question, you could make the room the parent of the custom command (right click the room >> add command).

Let's say the object is a chair. In the command script you would put: sit; sit down; sit in chair; sit down in chair in the first box at the top. And then run a script that prints: You sit in the chair (or whatever)