The act of entering a partner donation is just like entering a donation from any other org. However, there if you are required to assign the partner donation to one or more owner/partners (e.g. FEC), 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 done this, as soon as you begin to enter a donation for this org, the system will notice that you're entering a donation to a partnership and apply special handling.
Next, you must have owners/partners defined for that partnership before you can assign donation amounts to them. To do that, you must go into the Org Contacts tab (Orgs & PACs) and create a new contact for each owner. Be sure to select owner = yes. Optionally, you can assign an ownership percentage.
Note: If you already have an org contact be sure it is linked to a record in the constituents area. It usually will be. There will either be a link to the person's individual record, or a link to connect the record.
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 append your contribution-entry form with an ability to assign the donation to any of the known partners.