1. Import all contacts from a spreadsheet, taking care to not create duplicates and update any contact records we already have in the database. Probably best to tag these records to make them easily findable.
2. Search for these same folks as memberships, export any memberships that already exist with the membership ID (not contact ID).
3. Update spreadsheet to include membership ID—this is the only way Civi will update an existing membership in the system, otherwise it will just add a new membership which we don't want.
4. Import memberships in two parts: part 1 to update existing memberships and part 2 to add any new memberships.
5. Search again for all memberships, this time we should find every person on the spreadsheet. Export memberships again, making sure to include the membership ID (not the contact ID)
6. Update the spreadsheet to include the membership ID as a field on each contribution.
7. Import all the contributions, include the membership ID in an existing or custom field.
8. Run a script (custom development) that pairs each contribution based on the membership ID and the same value stored in a field on the contribution.