importing customers and orders

Can anyone tell me how to import all my customers and orders to my zen cart store with my backup file please? I'm using version 1.3.7.1.
Thanks

Import to where?

It is normal to use the complete Database and if changing versions of ZenCart, upgrade the database

I have some old databases that I'd like to import old customers & info from after doing fresh installs.
My skills have improved now & I am comoftable exporting & importing certain tables via sql & would like to get my old customer data integrated. Which tables are necessary & are there any major changes to customer related table structures over the years.

This is difficult to just import especially from 1 to many old ZenCart DB's
Each customer receives a unique ID#, and trying to just insert/add tables from elsewhere there will be duplicate ID's that will create many issues...
Doing this to a completly test install one might have a go at trying to get each table that has anything to do with "customer" exported as a csv from each source so you can insure that the ID's are unique...
Then insure that this is replicated across all other customer tables