I had two quite inexperienced NARs last night, maybe 10 games on the line between them. There are a lot of things to talk about - but I made the effort not to talk AT them. And I strung it out so I wasn't bombarding them. And I got them talking - I wanted to know if they were competent and I got a good idea of their weaknesses by asking them what they want to improve on. I only had 30 mins pre match at this level so some economy required
Personally I always start with the things that are specific to me and for the division and for these teams - usually in the dressing room. The things that they won't have heard before. They get to know me a bit and we get to talk about the teams and how intense we think the match will be. (Specifically, how to handle argumentative players as an AR, and how to handle mass con and, in particular the benches if there is a mass con.)
Before and after the warm up we go into the on pitch things. I always finish with goal/no goal and the importance of remembering to get to the goal line as appropriate to make decisions - because I think that is one of the hardest things to do naturally as an inexperienced AR.
For beginners, the message I bang home over and over again, about every aspect, is to take your time. Wait and see, look for me, signal together, think first, nice and calm, nice and neat.
The things I leave out for beginners are in the box/out of the box decision signalling and signalling cards. These are very advanced IMHO. I tell the ARs that any important decisions like this I am going to come and talk to them if there is any doubt. I want them focused on basic decision making - ball in/out, offside, fouls with me - and overall concentration.
There are some incredibly detailed pre-match lists on this site (search for Brian's).
