Let you RDO know you are interested, speak to the "higher" leages in the area - probs inter county level - and express your interest to their Ref sec that you are reffing in league X and that you would like to AR at their legue.
Also attend anything that is AR related in your area. CFA meetings etc sp you are both telling them your interest, and doing the leg work too.
Could alsp be worth trying to get into an academy in the area (if there is one) so you can do the line there and ply your trade.
Youth games are perfect for learning to line and perecting the signals, running, keeping level etc as are on smaller pitch.
Worked this end, and got a final last year AR, at a Ryman ground and 400 + spectators.
But you MUST keep your reffing up to maintain the AR. Rule being that you can AR at a level above where you ref, but you will still need to get your minimum number of games in per year in the middle to keep at the level / going for promotion. Down here it's minimum 20 a season middles, in a 32 week period, so if you only do saturdays it can get squeeky if you try and fir in both.
Personal avice - get you L7 sorted, then do your 7 - 6 so you are a bedded in ref and got the experience of a game and how it all works, THEN start going for your AR appointments as no doubt the advance of the AR side will imede on your progress as a Ref.
i belive that when you get to 5 / 4 then you MUST do so many lines as part of the promotion criteria, so it will come either by choice or necessity.
Oh and book to go to conference and choose the AR apecific workshops..... Kev Child, Darren Englind, Sean Massey and Mike Mullarkey give great sessions!