Hot air/hot plate/toaster oven soldering with solder paste are generally much easier than soldering by hand for circuits with more than a few SMD components. And a soldering stencil to accurately place consistent amounts of solder is much easier than laying down trails of solder with a syringe -- and there's much less board cleanup of solder bridges to do when a more controlled amount of paste is applied.
Unfortunately for those of us who prefer to etch a few proto boards at home when possible to test out a basic design & build quick development boards, stencils generally cost $35 or more and take a few days to get back. This is a way, using the same tools as etching circuit boards, to build quick proto solder stencils. The quality probably won't live up to the stainless steel or mylar ones you'd buy, but you might be surprised.
Note that the same method can also be used to chemically mill other designs -- decorative pieces for jewelry boxes, shadow designs for projecting with a Luxeon, etc -- possibilities are endless.
This method as posted won't eliminate the cleanup work, and I'm sure there are refinements to this method that will make it all work that much easier/better. I look forward to comments from others on ways to improve the method.
Apologies for lack of pictures in the later part; to get the pictures here I did a quick run-through but didn't do the actual etch/solder paste application. Poor quality pictures are due to close shots with a cell phone camera.
Step 1: Before we begin
The method here is based on how I do circuit board etching -- which is likely very different from how you might do it. I've been etching circuit boards for a few years now and how I do it has changed quite a bit over that time as I find new tools & approaches. Almost any standard way of etching boards should work fine for doing this too; whether you do iron on, light sensitize boards, etc; just a bit of thought and you should be able to adapt this to work with your methods.