1- Open up affinity designer.
2- Open up your image/project.
3- In layers, select the part of your image you would like to add the filter effect to in the layers tab.
4- Click on the effects tab in layers. a new drop down box will appear.
5- You can now select the effect filter by checking it.
6- Once checked you can now adjust the opacity, radius and even colour of your effect. There are many different filter effects you can use.