On the main toolbar, go to Export - Gerber and the Artwork Control Form window will appear. With this window open, select the view for the layer you want to create the Gerber file for in the main window. Then in the Artwork Control Form window, right-click a folder already present in the Film Control tab under Domain Selection and select Add. Launch a.pcb file, or any other file on your PC, by double-clicking it. If your file associations are set up correctly, the application that's meant to open your.pcb file will open it. It's possible you may need to download or purchase the correct application. Gerber files are the industry’s preferred file format for manufacturing printed circuit boards. Most PCB design software has the ability to output the design data in Gerber format to represent a design’s copper layer images, solder mask, legend, and drill and route data accurately.

Generate Drill and Gerber Files

Select File -> Plot from the menu to open the gerber generation tool.

In general, there are 8x layers you need to have a PCB fabricated:

• Top Copper (F.Cu)+ Soldermask (F.Mask) + Silkscreen (F.SilkS)
• Bottom Copper (B.Cu) + Soldermask (B.Mask) + Silkscreen (B.SilkS)
• Board outline (Edge.Cuts)
• Drill file

In the Plot window with the Plot format set for Gerber, be sure these Layers are checked:

If you don't know which layers, please check all layers, JLCPCB will help you to use the right layers. Click ‘Plot’ to generate the gerber files for the layers.

Don't forget to Generate the Drill Files. Click on ‘Generate Drill File’ button. You can use the defaults here as well. Check the 'Merge PTH and NPTH holes into one file' box. For now just click ‘Drill File’ or press ‘enter’ to generate the drill file. Use the same output folder as for the gerbers, which should be the default.

Express Pcb Gerber Files

Check the Gerber files in GerbView

Next, click ‘Close’to exit the Drill and Plot windows. All of the files should have appeared in your gerbers folder. KiCAD comes with a gerber viewer called GerbView, you can open the “GerbView” and check what your board looks like before sending it to manufacturer.

Gerber Pcb Design

How To Open Pcb Gerber File System

For more details, please see this video tutorial of generating Gerber files for manufacturing in KiCad.
For Kicad 5.1.5, please check this tutorial.
If everything looks OK, select all of the files, zip them up, and upload the zip file to JLCPCB order page.