There are 2 ways to filter data on a Custom Dashboard. One way is to create a filter element visible on the dashboard. These can be used to adjust the filter whilst viewing the data.
The other way to filter data is to use a filter in your query that calls data from the database. These can be used to make your query more efficient as it has to pull less data from the database. (Filters data out before it is displayed on the front end of the dashboard)
Firstly open a Dashboard, navigate to 'Reporting' > 'Dashboard Design' > 'Custom Dashboards'
Select the Dashboard you wish to edit and then click the 'Show Dashboard Designer' button. 
Adding a Query Filter
In the Dashboard Designer, navigate to the 'Data Source' tab and click on 'Edit'.
This will open the 'Query Editor', click on 'Run Query Builder...'
In the 'Query Builder' click on 'Filter...'.
This will open the 'Filter Editor'. Here I have created a filter to only show documents that are Sales Orders. Click 'OK' to apply the filter and then click 'OK' in the Query Builder and the finally, click 'Finish' in the Query Editor.
Adding a Filter Element
In the Dashboard Designer, navigate to the 'Home' tab. Select 'Filter Elements', lets add a 'Date Filter'.
To get the Date Filter to filter on Documents, we must add the 'DocumentDate' filed as the dimension on the Date Filter element.
You can use this filter whilst in the Dashboard Designer as well as when viewing the Dashboard from the front end. Click 'Set Filter...'
This Date Filter allows us to pick 2 dates in order to show all records that fall between these 2 dates. Here I have simply selected the year 2022 for both, to show all documents from 2022. Click 'Apply' when done.
With both the Query Filter and the Filter Element applied, this pivot now only show Sales Orders from 2022.
Additionally, you can navigate to the 'Design' tab, whilst the filter element is selected, to find options to change the way your filter element acts.
