Use a glycerin/stain liquid if you're using a powder stain or thin out your paste stain. Your veneer surface should have a matte finish as after grinding and steaming clean or pumicing clean or lightly sandblasting.
You're just trying to get rid of the surface tension so the stain will stay flat and not bead up. From your satin mixing pallet, you can either pick up some stain from the edge of a thin blade (image below) or thin brush and run it from the Gingival to Incisal, that way you can brush the excess (usually from the starting point) down away from the Inc, instead of trying to get rid of any excess right at the Inc.
You can also use a thin brush with stain on it, and do the same thing. Whatever technique you use, you can then take a clean wet thin brush, and run it up or down the side of the stain, gradually getting rid of any unwanted thickness, also bending your line a bit, so it isn't exactly straight up and down. When you're satisfied, run a bake about 20 C degrees lower than your glaze bake, as you're just setting the stain. Don't try to do it all in one shot, it may take you 2 "bakes" to get it there. If you really wanted to get fancy, you could run just a bit of violet next to your set white crack, as it gives it a look of translucency.
Also, don't use a pure white stain, if needed mix it with a little vanilla, A1 or light orange stain, to warm the color up a bit, so it isn't stark white.
I've seen others here post some really nice work with cracks, they may have some better ideas then I suggested.