Read up. The first stage in finding a suitable ERP system is to Read up and Research. Spend a lot of time in assimilating as much knowledge as possible. Google is your friend
Then Shortlist some vendors. Again research their products properly. New is not always bad. Old and experienced is not always good. The product is what matters.
Be aware of costs for the base product and for customization. Many vendors quote a very small base price and then make bank with hidden costs. Take your time finalizing, but once a vendor is finalized stop looking at other options. It can be too stressful. Trust your decision and go with it all in.
Spend a lot of time in documenting and signing-off requirements. Be aware of your scope of work. Don't compromise by accepting less than the agreed features. Equally, don't bully the vendor by asking more than agreed.
Start using the system. The best designed system will still have bugs, that's what updates are for. If you wait till everything is perfect before starting to use, you might as well not invest in an ERP.
Document every process with details including flowcharts, images and screenshots. If the processes aren't documented and followed, it defeats the purpose of an ERP.
Have management buy in at every step, and ensure there is a push from the top to use the system. Many employees are averse to change, and may try to avoid using the system.
Do not make ad-hoc decisions and exceptions to your own processes. It is simple to allow an exception, but over time the process is destroyed if there are more exceptions than rules. Take time to finalize a process, but once finalized stick to it.
Stay in constant touch with the vendor even when the system is fully implemented. They may have new features that may still help you. If there is an AMC, pay it to enjoy all new developments.