How to Make a Solder Paste Stencil
Most people use through-hole components for their projects because they are easy to use when bread-boarding a circuit and then easy to put together when you make your final project version. The problem that occurs, however, is that you cannot get many of the newer chips in through-hole packing either because they are specialized, used mostly in commercial products where SMT is common, or because the pin density is high.
The barrier to designing your circuit, doing the board layout, and getting boards made with SMT is not high. Eagle, for example, supports these devices easily, and you can either make your own boards on the PCB_Mill PCB plotter, or have it fabbed at any board house. You can even breadboard with through-hole components and then later do another variant by substituting the SMT version of the same components from the Eagle libraries (assuming both variants exist, of course).
When it comes to populating your own boards, you then run into potential problems. If you're only making one board, and you're using larger SMT components, you can use a magnifier and fine-tipped soldering iron to build your board. If you're doing a bunch, then you've got quite an effort ahead of you. One way to reduce the effort is to make a laser-cut solder paste stencil, and the great news is that we have everything you need at i3!
First, you'll need to export the stencil data from Eagle. In the board editor, turn off all of the layers except the Dimension and tCream (or bCream if you're putting the components on the bottom side). Then change the cream layer to a non-filled color. What you should see is the outline of your board, circles where you've placed any mounting holes, and a bunch of (usually) rectangular outlines where your pads are located. Print this view as a PDF with 1:1 scaling.
Import the PDF into Inkscape. You might be surprised to find that very little of what you saw in Eagle shows up. So change the Object Fill and Stroke to no fill, solid stroke, and 0.05mm stroke width. Then everything should show up. Save a copy of this in SVG format. Then Save a Copy in DXF format. Uncheck the lower selection in the DXF save menu and save the file someplave where you can find it easily, such as your desktop.
Open LaserCut (You might need to coop to a thumbdrive and take over to the laptop by the laser engraver). Import the DXF file. You should see the board outline and all of the pads and holes that you saw in Eagle. Select the board outline and make it a different layer. Then select any mounting holes and make them a third layer. Change the layer that holds your SMT pads to Engrave, and set the speed power to 400 / 25.
You'll need a sheet of polyester film (e.g., Mylar) for the mask. 3 or 4 mil thickness is typical for a solder stencil. You can use overhead transparency film from an office supply store, but make sure it's the kind for use in a laser printer / copier because it can withstand the heat. If you do not want to cut the mounting holes (you likely don't) or don't want to cut out the board outline (you likely do!), turn those layers off. If either of those layers is on, set it to Cut and set the speed power to 300 / 25.
Place a piece of thick (28lb or heavier) paper underneath the film on the laser bed. Align the sheets and find weights to hold down one of the edges so that the film/paper does not twist or shift. Run the Test on the laser engraver to make sure you've got it aligned right and that the head will not crash into the weights. Also, make sure that teh focus is set for the top surface of the film/paper so you get the best resolution. Then start your cut.
The paper underneath the film helps give you a better cut by preventing the laser from reflecting off the edges of the underlying honeycomb on the laser bed, so make sure you include this. Do not use tissue! The film will stick to the paper where it's cut / engraved, so you need a paper backing that's heavy enough to peel off from the film without tearing.
Note: If any of your parts have a very fine pitch (close spacing of small pins), you may find that parts of your stencil fall out, such as the center area of a chip. To avoid this, you can reduce the size of the pads slightly in Eagle. When you run your design rule check, change the SMT pad clearance on the Mask tab to allow a 1 or 2 mil clearance. It's counter-intuitive for this setting to be on the design rule check dialog, but that's where you can make several global settings for clearances and pad sizes for your board.