For starters, my experience is that the chocolate goldgrain is the preferred munchie of Revenue Auditors.

. That being said I have encountered some who would not even accept a glass of water ( for fear of appearing too pleasant or seeming under a compliment maybe )
Seriously though, if you have an advisor, accountant, auditor then they will have experience of this and can guide you through the basic process. If you are flying solo on this, then the most important things would be :-
Have a look at the Revenue's for Revenue Auditors
Make sure you review the accounts and records of the year(s) concerned and all relevant tax heads under review to see that you have all necessary information / records to hand.
If you become aware from that review or are already aware of issues which may mean tax underpaid then you need to look at making a disclosure before the audit gets under way. The RA will ask at the start is there anything you want to tell me now and that is your cue. Penalties for early disclosure are less than those which come from the RA digging it up themselves.
Do not be afraid to ask the RA to explain anything that you do not understand and, if in doubt or if the audit looks like it cannot be concluded in the one session, ask them to write down the issues they may have problems with and tell them that you will review the matters and get back to them.
Other posters may have further bits of wisdom to impart on this, sometimes nervewracking, experience.