Here are some examples of masking I've done to go along with my YCYW challenge.
Basically masking is covering any part where you don't want ink/paint/marker etc.
In the first card, I used letters where I didn't want ink. I put the letters down with some adhesive. Tip: if you don't have any repositionable adhesive, use permanent but stick to your clothes a couple of times to get some lint on there & it won't stick as bad to your paper.
Stamp/paint/mist all over. Then remove your letters. I used a gel pen to outline my letters so they were easier to see.
Another technique for masking is reverse masking. This time I left space where my letters were but covered the rest of my card.
I used a makeup sponge & ink so I could really get into the letters for this. I also inked the border punch. Remove your mask & you're done.
Hope this helped anyone with questions on masking!