I run a small Electronics design and retail company. I specialise in producing working prototypes from a concept, handling all aspects of schematic design, PCB layout, manufacturing the prototype and writing any micro-controller firmware. I hope people who read these guides will feel inspired to try some of the trickier aspects of electronics, it's not all magic and high tech - you can make some amazing designs by hand with some care and practice!
Surface mount soldering is not as tricky as some people make out - provided you have a good PCB design, with a correct solder-mask layer, and follow some simple rules and methods. I hope to make this easy.
There are three critical layers of interest regarding the actual PCB design; that of the copper, the solder mask and the paste mask. The first two are physical layers of the PCB, and dictate the PCB land (where the solder contact will be made) and the constraint of the soldering (to manage solder bridges, and other things) The paste mask layer is used to create the paste mask, which is typically a thin metal sheet, that is often laser cut these days.
This metal mask is used for a simple silk-screening process, where liquid solder paste is pushed through the mask (or screen) using a swipe of a metal blade. This process is also usually automated on the production line.
For prototyping needs, we don't need the expense and long-life of a metal screen, so recently it's been possible to buy solder stencils made from thin kapton sheet, using various web sites, for a very low price. These are still laser cut, so have excellent precision, just will wear out quickly. But we don't care about that :)
Update:- The pictures have lots of extra captions and information - so click on them and scroll through the sets!
I'm assuming you already know how to make a PCB design that is correct for manufacture, and have sent the gerbers, received your bare PCB, and it is correct, with a viable soldermask for the components you intend to solder.
The next step is to generate the appropriate data set for the solder paste stencil manufacturer. Recently I tried OSHstencils, so I can show you how easy it is to prepare the data in the format they need.
OSHstencils make a big deal about not including the board edge cut in the paste layer (presumably, because it's a waste of laser time, and could make the mask bigger than needed) I use KiCAD for my designs, so in the 'plot' dialog, you just need to select 'Exclude PCB edge from other layers' option, and provide three separate gerber files, one for each PCB side paste layer, and a third one containing the 'edge cuts' as a reference. Plot those layers, load them into gerbview (from the KiCAD main window) and check the data contains just the paste layers you expect, without the edge cuts. Login into OSHstencils, upload the data and check it looks correct in their web preview, and make your order.