Write your requirements for the system in plain English bullet points. Give this to vendors.
Write a plan of what you would like to see on the demo. Btw you want to see the boring stuff, not the colourful charts. Book a whole day. Take a person from purchasing, accounts, sales. Get them to create a sku and a supplier, get them to buy it. Get them to receipt it, get them to show the accountant how that affected the ledgers. Move it sell it, collect the cash etc. Now you know something about whether it will fit and whether you rate the vendor.
Could the company afford it costing twice what you planned? If not then maybe ERP is not for you.
You need a test system.
Allocate a lead person for every department. Let's call them the power user.
At key check points the vendor should want you to do testing. Write a test plan for end to end transactions like my demo example. Get your power users to do the test, this gives you buy-in, sign off and critically, job specific user training.
Allocate a huge sum of money for user training. Trainers are expensive. Job specific training is the only thing of value. There are often several ways to do something, only train in the one you have chosen.
The trainers need to be on-site for all of go-live week. Budget for this.
Don't train too early
Go standard, don't customise, work the way the ERP wants
On a phone but this feels like it would make a blog post!