Hello! Titan editor Andrew James has been a fan of my comic for years and we’ve spoken and kept in contact in that time. He approached me in November last year about working on Who and it took a lot of testing and trying out strips before we all locked down what the back ups would be like.
Titan do have a submissions guide on their website http://titanbooks.com/footer/submissions which I didn’t look into as they approached me but my advice for those wanting to get work in comics is simple: Make comics. Write them, draw them, go to small press fairs and sell them, put them online, whatever just make make make make. Show them to editors, to creators you admire, throw them at retailers - ask for advice and don’t be disheartened, it took me 8 years to get a gig like this and I’m holding onto it with an iron grip.
Hope that’s useful!