With some basic craft supplies and a printer you can do a lot! My favorite method is using canvas and stencils. All you need to do is find an image of what you want on your canvas, an x-acto knife to cut it into a stencil, paint and a canvas. Just trace your stencil onto the canvas and you're good to paint it however you want.
Or, if you're better at drawing than I am you can just skip the stencil entirely... You can also use painters tape like I did for these Ravenclaw and Gryffindor paintings.
Another method is using quotes. All you need is a collection of your favorite quotes, printed & cut out, a canvas and mod podge. Just fit the quotes to your canvas so that there are no gaps, then mod podge them on. While I did them in black and white, you could also print them in color depending on how you want your finished product to look.
Another fun decor option is printing maps from your favorite fandoms. I have 4 hanging up right now that are just on stock paper, but they would look so much better framed. Maybe eventually...
How do you decorate your house while expressing your love of your favorite fandoms?