These steps assume you are creating your Reserve Fund as an equity account. However, you could also set up your Reserve Fund as a liability. The steps to accomplish that will be the same, you'll just create the account as a liability instead.
1) Go to Budgets > Edit Categories > Equity > Add Equity. Give the fund the
appropriate name, such as “Replacement Reserves,”
the date you want that fund to start showing on reports
and the amount of the fund
*Values are only an example, you should input your own data into these fields
2) You can then go to Reports > Balance Sheet > Update Reports. Once those are updated you will see those new equities on your balance sheet.