I have been studying the mechanics of the pegasus Stargates due to the work I'm about to start doing on the new sim gate screen.
The dialing procedure involves that the chevrons of a Stargate must be locked. This entails the symbol that was dialed appearing on the symbol ring under the last locked chevron (or under the top center one for the start of locking chevron 1). It then moves either clockwise or counter-clockwise around the ring until it reaches the next chevron. The chevron lights up and then the next symbol appears under it and rotates in the opposite direction. This carries on until all seven have been entered and if successful a wormhole is opened to the destination gate. The rule of thumb seems to be if the first to be passed on the next rotation is the actual target chevron then skip it and carry on rotating until the 2nd time it is reached at which point stop and lock it.
Heres a cool snippet by tpg1989 of the gate dialing.
I put this short Flash demo together to show the full animated rotation and the following are some static step by steps.
1st is counter clockwise 2nd is clockwise 3rd is counter clockwise 4th is clockwise 5th is counter clockwise 6th is clockwise 7th is counter clockwise