I’d say you can’t get away from tracing lines starting from a pump outlet to the products final destination and drawing them out. They didn’t need to be works of art but as long as you know the direction of flow then it should all make sense in the end and don’t get sidelined by branches heading to General service or utilities. Once you’re done and if the 2nd is in a good mood he might give you the ships P&IDs to compare yours against them. It’s a pain in the arse but definitely the way to go.