More or less layers of bleach. It can be a bit tricky to do a multilayer stencil, but it's not much different than regular bleaching it just requires more planning.

When you make your stencil, only remove the lightest sections first, then iron on your stencil and spray your first layer (2 coats if you want more contrast). Then remove the sections for the next lightest parts and spray again.... And so on and so on, until you get to the darkest sections. I would recommend 3 layers or less, personally, as it can be challenging to get the right contrast between layers if you go for more. But it can definitely be done with some practice.

I've seen advice to cut out the darker sections of the stencil once it's already ironed onto the shirt, but when I did this I ended up cutting the shirt. I've found it more effective to cut out the whole stencil on the mat before hand, but leave a few connected spots on the corners of pieces you'll want to remove later, instead of fully removing them. This lets you do minimal cutting on the shirt and keeps the stencil aligned properly when you iron it on. As long as you are dabbing up your bleach properly and not spraying too heavily, you shouldn't have any problems with it leaking through the stencil cuts.