The act of entering a partner donation is similar to entering a donation from any other org. However, if you are required to assign the partner donation to one or more owner/partners, then there are a series of prerequisites before you can enter the donation.
First, you will need to have entered the organization into your Orgs and PACs area and you will have to assign it the org type = business and business type = LLC/Partnership.
Once you've entered the new org as a partnership, you need to add the owners/partners for that new org. Go to the Org Contacts tab and create a new contact for each owner. If the persons are already in your system (as Consituents/People) you can just select them. Check the box for owner = yes. Then assign an ownership percentage for the partnership.
Once you have a partnership defined, with owners, you can enter the donation as normal. The system will recognize that you are entering a donation for a partner and automatically distribute the funds of the donation corresponding to the ownership percentages (which you can override if you wish).