Well I know it's an obvious place to start, but make sure you listen to plenty of dub. King Tubby comes highly recommended from me! Bass and drums are the foundations of dub and they both take practice to get the feel right. Generally the hats are in a swung, triplet rhythm. Kick and snare are on the 3rd beat and slightly late. You can have the kick on all four beats for a 'steppers' style or on the 1st and 3rd which can work well too.

Bass is usually late too and often doesn't start with the first beat of the bar. Generally you want to keep things simple with 5ths, 3rds and landing on the root (the heaviest note) on the 3rd beat with the kick drum.

The skank is next which comes in on the 2nd and 4th beat, again slightly laid back. You don't want to have the skank in all the time, but bring it in and out strategically with some massive reverbs and delays. Everything other than the drums and bass are decorative elements in dub.

Then horns, vocals, keyboards. All in small amounts staying minimal and usually with lots of reverb and delay. Try a cavernous reverb on the snare every so often and phasers on the hats. It's all about experimentation so try new things all the time!

I can't really describe dub in the box because it's all about being out of the box, but hopefully there should be a few decent basics here. The feel NEEDS to be right and you should feel pulled into the 3rd beat with the skanks inbetween lifting you up. It's a hard thing to describe but you'll know when you've got it right when you turn it right up and it makes you want to groove!

Good luck man