Importing Memberships and Contributions but can't seem to link the two together
The key to my instructions are that there is some unique identifier in both your memberships and contributions.  If you don't have one, then you're correct, there's no way to link the two.

That said - you can use some trickery to create a unique identifier after the fact.  For instance, if your import data has a contact ID and a transaction date, and you know that one contact never signs up for two memberships in a day, you can create a unique ID based on the data you've already imported to the memberships.