A riso print is made using a risograph machine which looks a bit like a photocopier, using a process a bit like screen printing.
For each individual colour you create a different greyscale design (called a separation), or you can place anything which could be photocopied on the glass of the machine. The machine then punches the design through very thin paper to create a master. The ink from a large ink drum is forced through the master to create a print. It takes a few prints to get the ink flowing.
To add another colour, you first change the ink drum. Then change the separation, before creating a new master, reloading your paper and printing.
I love the experimental nature of riso printing, I use photographs I have taken and textures I have collected to create my prints. There is always a bit of slippage in the process of printing, which produces unexpected and often beautiful results.