If you have someone else doing the League, you can get a backup of the league from them and use their setup.
Otherwise you create a league the same as a League Secretary with just the required information: Name of League, Number of Teams, Bowlers per Team, Start Date, Number of Weeks, and Type of Bowlers. No need to change any other rules (the switch for "Manage Treasurer duties" should already be set to Yes). When you get to the lane assignment screen, you only need to make sure that the number of weeks are correct. After you get the League Rules setup, go to the Setup/Teams/Rosters and create the teams and enter the bowlers.
At this point, the League portion is setup as you need it to do the Treasurer. Continue with the Setup/Fees/Weekly to set up your fees and then you will be ready to go to the Weekly/Enter Weekly Bowling Fees to begin your first week of Treasurer duties.