Contact Cricket Australia and get on the umpires course.
In England the Level One course covers the Laws and the One A covers fieldcraft, game management etc. You then go on to be assessed on the field.
There are then multiple choice exams.
When I qualified it was just one intense course followed by a long written exam, an oral exam and on pitch assessment by an experienced colleague.
I would observe that a bad back is no a help - you will feel it late in a long day.
The first thing you will hear on the course (it might be you saying it) is "'I played for xx years, I know all of this."
The second thing you will hear is "Oh, I didn't know that."
The two trickiest things are the small changes that are brought in to the Laws at times - if you don't hear about them - and the things that happen very, very rarely. With the latter hopefully you or your colleague have it right. (note: if you have a qualified colleague the ability to work closely with them is very important - players do not see how much this goes on).
A few key points:
Always stay calm. Even if you have to hold up play an take a couple of seconds to compose yourself do not rush a decision.
Every delivery is a new event. What happened on the last ball, or over, or hour doesn't matter.
Never assume or anticipate what you think will happen before it does (actually,there is one thing in the Laws umpires do 'assume' - I think it is wrong).
A quiet word or two in the ear of a player, or captain, in passing can often diffuse something that might otherwise develop into a problem.
Certain players will chat freely, others will not. Some won't talk at the start of things, but will as the game goes on. I will, for example, normally say hello to a new batsman - some respond others don't but do not force it. A quiet "unlucky" to a bowler who is having no luck and getting annoyed can calm him down (and helps when you then turn down one he reckons is plumb). Some quite high level colleagues I know don't interact with the players at all - I think that is wrong.
I've said to players, including some who have or will play, or do play, FC cricket "It is the player's game, not the umpires. If you want the game to be played in a friendly spirit I will assist in that. If you need me to be 'in charge' and officious then I will be". They like this
When you are umpiring regularly and meet other umpires you WILL swop stories of incidents and moments in games.This is very useful as it often prepares you for things that you haven't seen that may come up.
Batsmen are never out LBW, bowlers think every one is plumb.
Trust your judgment an never second guess yourself on the field. But if you make an error and realise straight away don't be afraid to revoke your decision.
Just a few things.