1) Log in to server UK1.
2) Click the 'Online' button at the top of the screen
3) Click 'Create a game'
4) Click the 'Advanced setup' button
5) Click the 'Generate Extermination' button
6) If you get a silly map, or if you don't have four men per side with at least one explosive type, repeat the previous step until you're happy.
7) Click 'Save level' then enter a meaningful name (I'd suggest your opponent's name) and click 'Okay'. (If you forget this step it' possible to recover the map so just ask if you have any trouble and someone will be able to help.)
8) Click the 'Continue' button
9) Ensure that the 'Game mode' is set to 'Extermination'
10) Ensure that the 'Vis mode' is set to 'Vis at time zero'
11) Ensure that the 'Turn limit' is set to 8 and the 'Turn length' to 5000.
12) Activate Mutators, if you like (although you'll only be able to do this if you own the Red DLC)
13) Enter your opponent's username (and select "I play side 1" immediately below if this is the first challenge to this opponent)
14) Enter 'RPS League' or something suitable so that your opponent knows it's for this league
15) Click the 'Submit' button
16) Take your first turn and note the challenge in this thread, using the format in the post directly above this one
Then, to start the mirror game, repeat steps 1 thru' 5, then:
17) Click 'Load level' and enter the name of the map from step 7
18) Click the 'Continue' button
19) Change 'I play side 1' to 'I play side 2'
20) Go to step 9 above - don't forget to make the Mutators the same, it's easy to overlook
If anything goes wrong, don't worry! Just issue the correct challenges and let your opponent know which is the correct game so they can ignore the faulty ones (which will eventually be deleted from your list if no-one takes a turn, and can be hdden until this happens).