County or League semi? Sussex run AR training nights for all the folk they've assigned to run lines for their semi/finals - check whether your CFA does or not.
Otherwise, my five top (?!) tips:
1/ Practice crabbing, if for no reason other than to get your leg muscles used to it.
2/ Hold the flag in the hand between you and the referee - this will most often be your left hand, but if running up the line towards the centre may be your right hand.
3/ Be crisp with your signals.
4/ Don't change hands whilst the flag is extended.
5/ Mirror the other NAR if required, if the flag needs to be brought to the attention of the ref (i.e. if the other NAR is flagging and the ref doesn't see it, you flag as well).
Can I sound rude and not mean too
Point 5 is not taught, or encouraged, maybe locally, but certainly not globally
And even then, too advanced for someone just starting out