I'm attempting to follow a Photoshop tutorial, but I'm having trouble creating the layer mask. I can create it, but I don't understand how to make the paint brush "erase" the layer below it. I don't see the little White and Black boxes that appear in photoshop to make the layer mask either visible or not visible. If someone could rewrite the tutorial below so that it works in Affinity Photo, that would be super appreciated.
I just need these 4 steps rewritten. I have also posted the original source article link:
Create A New Layer. Type Ctrl-J (Command-J) or flatten all the layers into a new layer using Ctrl-Alt-Shift-E(Command-Option-Shift-E).
Add Median Blur. Click Filter > Blur > Median. Then select a Radius of 15 - 30 Pixels depending on the amount of dirt and grime on the background. In the preview window, make sure the dirt completely vanishes, but the shadows retain detail.
Create A Layer Mask. Click the Layer Mask button and then tap Ctrl-I (Command-I) to invert the mask.
Paint On The Median Blur. With a decent size white brush paint on the Median blur over the problem areas and watch the dirt disappear! Be careful of any edges, I recommend using a small brush to cover the fine details around shoes or props.
https://fstoppers.com/commercial/save-yourself-hours-most-effective-way-clean-backgrounds-photoshop-32144
Thank you in advance.