Step 1: Prepare a .CSV file
Step 2: In Salesforce click on App launcher and search for NPSP Data Imports
Step 3: Click on NPSP Data Imports
Step 4: Click on “Import”
Step 5: Select Custom object -> NPSP Data import object
Step 6: Select Add New records and leave other settings as is
Step 7: Attach the prepared .csv file and Map the CSV fields with salesforce
Step 8: Click on Next -> Start Import and Ok
Step 9: You will see a result of records passed and failed. (If you get failed record read the error then make changes accordingly in CSV and start again from step 2)
For passed records Now load this all in Salesforce objects. (Passed records are still not in actual objects currently they are in the NPSP data import object.)
Step 10: Go to NPSP Data import from app launcher
Step 11: Click on Start Data Import
Step 12: Click on Begin Dry Run and then Begin Data Import Process. (Dry Run is optional)
Now all your historical data is imported to Salesforce.
NOTE: if you are using donationManager make the below-mentioned change to create Payment records.
Step 1: From App launcher type NPSP Settings and click on it
Step 2: From the left panel click on Donation -> Payments
Step 3: Remove “Donation” from the “Excluded Opp Record Types”. (So that NPSP will create a payment record.)