Capabilities

Make sure that JLCPCB can manufacture the PCB you want, their capabilities page lists what they can and can't do. At the time I am writing this, they handle up to 6 layer boards. They can't do blind or buried vias (but then again, most generic PCB fab places can't, and I'm hard pressed to think of a design in our applications where such space optimization would be necessary).

Account

I have made a 40m account - contact me or Chub for the login creds. It is linked to the foteee@gmail.com account, but their web interface already allows correspondence with their engineers, who are pretty helpful.

Common gotchas

Most times, you would just download the "Fabrication Outputs" zip file from the DCC for the design you want to fabricate. A common problem I have encountered is that often, the board outline is not exported as a Gerber file. This stackexchange details the fix - you have to do this in Altium. Then, you have to re-export the files, following the nice writeup on how to do this.