Select the logging button (via the "+" button at the bottom of the screen), choose the desired date in the date navigation bar at the top of the screen, then select the custom log category.
Click on the + sign to create your custom log. This log category will now be visible as a daily selection for you in the list of custom log categories.
How to delete a custom log:
For iOS, swipe to the left on the custom log you want to delete.
For Android, tap the little arrow on the right of the custom log you want to delete. Select delete and confirm your choice.
Once you have deleted a custom log, it cannot be recovered. You will need to add the log entry again.