For your pen and acrylic colour wash project here is the demo we looked at in class. If your pen is water based non-permanent it will bleed in interesting ways with the watery acrylic washes (in the demo we used permanent marker pen so this did not happen).
The demo painting shown below is at second stage, first stage is the loose marker pen drawing (keep it light touch and very simply - do not shade in areas, keep it as line only). The colour is mixed on a large flat palette with just a small touch of paint and lots of water. Use colour straight from tube or mix colours on the palette. You can even try using white as a colour wash over other colours. The paper is dry and not stretched.
Use a large long round hog, size 8 to 12 depending on the size of your paper. Apply light washes with very wet brush, working quickly and loosely. As you drag the brush it will dry giving different textures of paint. Do not try to paint inside the black pen lines, let the colour spread quite randomly across the lines.