What are the best accounting practices for Fx swaps?

This is a very large question. If you have not taken any action to designate a special hedge accounting relationship, you fair value the instrument at period end (any legs that have not matured) and record that as an asset (if a gain) or liability (if a loss) ont he balance sheet. The offsetting entry is to earnings in the period. I'm a big fan of reversing entries so I would reverse that at the start of the following period and if there are unmatured legs of the swap at the following month end repeat the process. Whe individual legs are closed the related gain or loss is hardbooked to earnings. So book the reversing gains and losses where you intend to book the final gain/loss on the contract. ASC 815 has all the rules on accounting for derivatives. The summary above is captured in ASC815-20-35-1a.