Hi! So I'm going to give you two different methods just bear with me.
1)
A) When you enter the app, there's three taskbars: there's one to your right, one to your left, and one up above. On the one to your left, you'll see the different brush options. Click on the brush you're currently using and it will show up with basic settings, which has two separate sliders: size and opacity. Modify the opacity slider and you're good to go.
B) This option can actually also be activated by modifying the two gray sliders on the left side of your screen. They are right on top of each other and vertical rather than horizontal. You can slide the BOTTOM one up to increase opacity and down to decrease. The one above it is size.
2) If you're trying to modify the opacity of an entire layer click on a specific layer on the menu on the RIGHT SIDE of your screen (one of the boxes) and a whole bunch of different small symbols will pop up, including ones that say "duplicate" and "delete". There should be, at the bottom of the box, a slider. Move the slider to the right to increase the opacity of the layer and left to decrease it. It will show you the percentage as well.
Ps. If you merge a layer that has a lesser opacity onto one with a higher opacity the colours will stay the same as you merge them but the opacity level will go back to 100%.
Hope that helps! Happy arting!
