PCBs are increasingly cheaper and easier to manufacture these days - from milling it yourself, chemical etching or using one of the numerous super cheap pooled manufacturing services (my favourite being DirtyPCBs). Designing your own PCBs means you can easily take advantage of cheaper and smaller surface mount components, but assembling them can be a long and tedious processes. For small-run batches of boards that don't warrant paying for a professionally made steel solder stencil, I generally use a syringe with a blunt tip to manually apply solder paste to each pad on the board. Not only does this take a long time, it leads to sore fingers and a lack of control of the amount of paste on each pad. Having inconsistant amounts of paste usually results in problems during reflow like tombstoning components and bridged IC pins that take a bit of time cleaning up afterwards. With a solder paste stencil, you simply lay it over your board and use a credit card to apply a nice even layer of paste on each pad in one go. Easy!
I'll explain the process of how to export your board design from Eagle, clean it up in Adobe Illustrator, then cut it out with APS-Ethos and our CadCam FB500 laser cutter. You'll need some Mylar stencil sheet - I bought some 125 micron A4 sheets from eBay for around 71p each.