To lock and close an Accounting Period first go to Settings on the left-hand menu of a client’s dashboard, go to the Account Period section and then click on Edit in the top right-hand corner.  

Then click on the Edit icon within the Action column for the Accounting Period in question. Then click the Lock Accounting Period within the Manage Accounting Period page which will then redirect back to the previous page.  

To close an Accounting Period, click on the Edit icon within the Action column again for the Accounting Period in question. Click the Close Accounting Period button. There is also an option to upload any additional files or documents within the Manage Accounting Period page.  

When closing an accounting period, it will carry forward the closing balance to the next accounting period. Therefore, you will need to reverse the opening balance sheet journal if the trial balance you are going to import for the next period also has opening balances in it. Here’s a video that explains how to post a reversal journal for the balances carried forward to next period by Nomisma.