Really hard to explain, but the best way to get a crisp, clean flag signal is to use your finger as directioner.
I hold my thumb on the buzzer button/where the buzzer button would normally be and then my index finger points along the flag pole. When I signal, I am essentially pointing at a 45degree angle and the index finger stops the flag from ****ing.
Seems trivial, but it looks so much more proffessional.
Always keep flag in hand closest to ref unless signalling, this is normally the left hand unless running up towards halfway and if the ref is using outside rights (lb/RW). Outside lefts is opposite but there are only very very few old schoolers who still do it this way.
Crab up and down the line when play allows, try not to walk to much.
Change flag over down below when signalling for attackers.
Make eye contact with referee before signalling to agree the direction. A good ref will be leading you in. Don't be afraid to tell him discreetly he is wrong if you are 110%. The ref should give you some input on what to do here anyway so listen to the pre match.
Most importantly learn from the ref. Both good and bad things. If you see something you don't like you can not do that in your game and of you see or hear things you do like then you can see how to incorporate that into yours.