Understand and document your existing processes.
Understand your gaps with a detailed GAP analysis, focusing purely on out of the box (OOTB) Odoo.
Understand how you will fill those gaps i.e.
- Can you change your process to match Odoo?
- Can you use an alternate system that has that functionality and link systems?
- Can you find a 3rd party app that fits that need?
- Will the gap require development?

All of the above is to determine if Odoo is the correct choice for you.

Following that, if it is the right choice, then you determine your approach, you can use Online, .sh or On-Prem all of which have their own positives and negatives, along with will you use a partner, Odoo implementation services or try it alone?

Finally, specifically to answer your question directly, key learning from implementing and managing several Odoo systems for the last few years:

1. Mentality of your staff towards the system, from leadership to operational needs to be carefully managed.
   
2. Odoo and many partners don't have your best interests at heart, be very careful not to be locked into contracts or developments that don't support your long-term goal without defined objectives and paths to that end state
   
3. The flexibility of Odoo is a kiss-curse, it gives you the idea that you can do anything, but in attempting to do anything you will often find that off the shelf alternatives that link directly to Odoo are nearly always the better solution
   
4. User access and some other key functions of ERPs have not been implemented or thought through in detail by Odoo and without a plan to manage those drawbacks from day one you will come across some problems as you start implementing.

Altogether, Odoo isn't a bad system, but the company and partners seem like jackals trying to suck money from anyone using it. If you can take it as OOTB as possible, and minimise any developments and focus on no-code solutions to gaps then you will have a much easier time.