When migrating historic donation records into a nonprofit's Salesforce org where many of these donations were paid in multiple installments, it is important to ensure that the Payments are correctly matched to the Opportunities. The correct approach involves using the "Do Not Automatically Create Payment" field:
Check the Do Not Automatically Create Payment Field:
When importing historic donations, check the "Do Not Automatically Create Payment" field on the Opportunities.
This prevents Salesforce from automatically creating Payments for these Opportunities, which allows you to manually enter the correct Payments that match the historic installment records.
Manually Create Payments:
After importing the Opportunities with the "Do Not Automatically Create Payment" field checked, manually create Payment records that correspond to each installment.
Ensure each Payment is correctly associated with the corresponding Opportunity.
Validation:
Validate the imported data to ensure all Payments are correctly matched with the Opportunities.
This method ensures that the historic donation records with multiple installments are accurately represented in Salesforce.
Reference:
Salesforce Nonprofit Success Pack (NPSP) Documentation: Managing Payments and Opportunities.
Salesforce Help: Data Import Guidelines.

In the NPSP Data Import Template, the Account fields should contain:
Household-related data:
This data includes information pertinent to the household of the contacts, such as household name and address. Household-related data is crucial for managing and segmenting contacts within the NPSP.
Reference:
Contact's employer-related data:
This data includes the employer's name and details, which helps in tracking affiliations and donations made by contacts through their employers. It is essential for managing donor relationships and corporate affiliations.

When implementing Salesforce Shield Platform Encryption with NPSP, there are several key considerations:
Role Name Field on Partial Soft Credit Object:
If the Role Name field on the NPSP Partial Soft Credit object is encrypted, the nightly Soft Credit rollups will fail. This is because the rollup process relies on accessing this field, and encryption can interfere with its functionality.
MergeContacts List Button:
The NPSP MergeContacts list button on Contact list views will fail if the Contact Name is encrypted. The merging process requires access to the Contact Name to identify and merge records accurately, which encryption can prevent.
Custom Unique Id Matching:
NPSP Data Import cannot perform Custom Unique Id matching on Accounts and Contacts if the field used for matching is encrypted. This limitation arises because the encryption prevents the import process from accurately comparing and matching the encrypted values.
These considerations highlight the impact of encryption on certain NPSP functionalities, and it is important to plan accordingly when implementing Shield Platform Encryption.
Reference:
Salesforce Shield Platform Encryption Documentation
Nonprofit Success Pack (NPSP) Documentation on Data Import and Soft Credits

Explanation
https://powerofus.force.com/s/article/NPSP-Install