In this system, 1NT is the only forcing opening bid. Opening bids are generally made with 12 HCP, approximately using the rule of 20.
Opening bid rules
You are allowed to open 1/ if you have at least 5 cards in the suit.
You are allowed to open 1 if you have at least 4 cards in the suit.
If more than one suit is allowed, open the longest of these suits.
If still more than one suit is allowed, open the higher of these suits.
Examples 5 3 0 5: Open 1, the highest of and .
5 1 6 1: Open 1, the longest of , and .
1 4 4 4: Open 1, the highest of and .
4 3 3 3: Only is allowed. Open 1.
Further bidding. Both players find out in what range they are. Opener's ranges are:
11 - 15: Weak
16 - 17: Medium
18 - 20: Strong
Responder's ranges are:
6 - 10: Weak
11 - 12: Medium
Weak hands try to stop as low as possible, medium hands make a game try and strong hands force to game. Opposite a game try, a weak hand tries to see if he has something extra, which is needed for the game.
Bidding with fit
Weak hands establish the fit on the 2-level, medium hands on the 3-level and strong hands bid game.
Bidding without fit
Responding to 1-bids
Weak hands have to stay on the 1-level, so they either bid a new suit (forcing) or 1NT. If you have a good 6-card suit that you could instead bid at the 1-level and a weak hand you can bid a Weak Two as response to partner's opening bid.
Medium hands can go to the 2-level, so they bid their long suit (forcing) or bid 2NT.
Strong hands bid their suit or bid 3NT.
After a possibly weak bid, the maximum level for weak hands is returning to the opened suit.
With medium hands, one can bid a new suit even if it is higher than two in the opened suit. If no suits are worth mentioning, bid 2NT, or 3 in the opened suit.
With strong hands, one can also jump to the 3-level to show a suit if it is lower than the opened suit. Therefore a sequence like 1 - 1 - 2 can be passed, although responder must be aware that it could have been bid on a medium hand. Suits bid on the 1-level are always forcing. One can also bid 3NT without anything else worth mentioning.
After a new suit on the 2-level (medium or better), the game try can be declined with 3 bids: Bidding one of the two known suits as cheaply as possible or 2NT. Anything else accepts the invitation and is forcing to game.