Example, you want the last word in clause 2 to be a reference to clause 1:
1. The first rule of fight club is you don't talk about fight club
2. See rule 1
Here is how to do it:
1) Place the cursor where you want the reference.
2) Insert -> Reference -> Cross-reference
3) Set "Reference type:" to "Numbered Item"
4) Set "Insert reference to:" to "Paragraph number"
5) Choose the correct reference from the list
6) Click insert
7) Click close
Now you can insert a new clause between 1 and 2 (so that clause 2 becomes 3)
To update the references, select the whole document (Ctrl-A) and hit F9