Stain

I need some helpful tips on staining and stenciling. Say I have a piece of wood and
I stain it a light color but then want to stain over part of it with something darker.

Do I use painters tape? Gel stain? Rag?

Walk me through this process please

If you are going to stencil with a dark stain, you will need to apply that first and then follow up with a light stain over the top. Once stain is applied, another stain over the top of it will make little to no color change.

you will have to sand down the area you want to restain...open up the pores of the wood so it will accept the new stain. Most stains these days alse serve as a sealant, so the wood's pores get closed off really quick.