One of my favorite techniques has always been to use Japanese paper. Now there's a lot of different variations of this type of paper and I am now looking for variations on the paper that I currently have because it's not available anymore from the vendor I used to get printmaking paper. But the kind that I love using right now is very airy, soft, super delicate, transparent, handmade and easily can tear etc. So stay tuned because I have to experiment pretty soon here with different kinds and I'm not sure I can find the kind that I love pictured here! But oh well. Have to move on! But I have always loved to draw over this type of paper and then to paste the paper itself into my paintings as these soft layers that give this beautiful translucent effects. You can control how "translucent" you let the paper get when you apply a basic medium to "glue" it to any painting surface. Add more water and it becomes more transparent.
I also highly recommend using your own photos. Here I have photo of flowers I took in England and I liked the composition enough to just play on top. I loosely drew my flowery sketches not caring at all if they were "exact." It's just a reference.
If you use other images, make sure they are general, like general patterns or other neutral imagery so you don't get into any issues with copying other people's stuff if you want to sell this painting. But it's really more fun to use your own stuff and get inspiration from yourself.
But the effect creates a loose, sketchy look like you really did draw onto the painting surface but also includes some kind of layered effect. Experiment with different paper and all different kinds of colors of pens, pencils, charcoal you name it. Sky's the limit!
In this series below you can see where I applied this effect.