The P&L of the partnership is the same as any set of accounts. You will need to deal with capital introduced, drawings and profit apportionment seperately for each partner.
The Form 1 is the partnership return and discloses the details of the trade to the Revenue.
Each partner has to file a Form 11 with their share of the partnership profits together with any other source of income.
If you are unsure ask one of the seniors in your office to look over the accounts, comps and returns for you as your doing a favour for your BIL. But be sure the everything is reconciled properly. PM if you get stuck.