First thing you want to do is decide what you would like your theme to be and how long you would like it to last, and if you would like to invite someone to co-host. Then ask your chosen co-host, and set a date for it. Advertise a few days in advance. Ask people to tag their theme week memes accordingly. "Me and the Boys Week" for example, or "Cat week". "Zelda Week" Dog Weekend". You get the idea there.
A word on the streams, You must search each stream separately for theme participants as we do not currently have a site wide search option. Because it is expected that the host and co-host of a theme week comment and vote on participating memes you will need to bear that in mind. It was difficult trying to search out memes from every stream for the "Me and the Boys Week" .