This has been covered elsewhere. Sparkfun has a set of particularly good tutorials
. Note that Seeed requests that you put your order number on the silkscreen layer somewhere, so you might want to consider placing your order before going through the render/check cycle.
When you're happy with your board, and it passes Seeed's design rules, use the Seeed CAM processor to generate the Gerbers for your PCB.
When you're done, you should end up with the following files:
<board>.GTO - top silkscreen
<board>.GTS - top soldermask
<board>.GTL - top copper
<board>.GBO - bottom silkscreen
<board>.GBS - bottom soldermask
<board>.GBL - bottom copper
<board>.TXT - Excellon drill file.
Before you proceed, you should check the Gerbers carefully to make sure everything looks right *before* combining them into a panel, this will save time later if you find a problem. I like to use the online gerber viewer provided by CircuitPeople
. You can make a zip of all the files and upload them in one go, and they will all be rendered to image files.