well nothing special, just another normal tunnel. assuming you want to acces a tcp port that is only reachable via a box which itself you can only reach via sshing to another
your client command: ssh -L 1234:localhost:2345 remotebox1
your ssh from remotebox1: ssh -L 2345:remotebox3:3456 remotebox2
so you then connect to localhost:1234 and that ends up hitting 2345 on the first box you ssh to. that traffic then gets forwarded from port 2345 on the box through the 2nd box and onwards to the third system which is runnign the service you actually want.
but then hang on... you can connect from your client to box1, that ssh connection doesn't necessarily need to do anything special as you can also presumably hit any port on the box, not just 22? assumign that that's the case, you only need to establish the tunnel on the 2nd command... "ssh -L 1234:remotebox3:3456 remotebox2" so you then connect to port 1234 on box1 (instead of the client machien itself as in the first example) and then direct to box 3.