As you know [ probably don't, but anyway... ], I love electronics, and getting your projects and design looking professional and mechanically stable is a great plus you get when assembling them in a printed circuit board.
When ordering PCB's there's a certain process you have to go through, from designing the circuit, to designing the board, then ordering and actually assembling the circuit. For this guide I'm assuming you know your basic electronics and are looking to order a batch of PCB's for your finished project.
First off, you need to have your project designed and tested. This is probably the hardest part of the process, since you need to be making schematics, some calculations and stuff related. In this process, make sure to have a dependable breadboard, power source and measurement tools around, because many times problems with the functioning of the circuit can be caused by irregularities in any of the equipment described.
Parallel to designing the circuit, you need to be making the schematic in a computer program, because you'll need them to generate the circuit board layouts. The program you choose is pretty much a matter of choice, since most of them work around the SPICE model and net lists, so choose one that you like or a friend recommended and get to work.
Next up is designing the circuit board. Before you start doing this, make sure to check with your manufacturer if they offer standard sizes, more often than not, these standard sizes will give you a better value for low volumes of circuit boards. Once you have the size, you can get started on the design of the circuit board. The actual designing of the traces and positioning of the components in the board is an art and science in itself, and that is outside the scope of this thing. Many top notch manufacturers offer tutorials on how to properly design a PCB [ they want to make their job easy too you know ].
Remember I told you you needed to have a PC version of the schematic?, well that's because you really should check, double check, simulate and re-simulate your circuit and confirm that all connections in the PCB design correspond to those in your schematics, because once you order, there's no going back [ except for manufacturing defects ], and circuit boards are not the cheapest thing to buy.
Now that you have your circuit and circuit board designed, its time to get them manufactured. If you took the time to check for standard sizes then this step should be very straightforward, you just select your standard order, upload the Gerber files [ which are the layout definition file of the circuit board ], depending on the manufacturer, you may need to compress [ zip ] the files before uploading.
Before purchasing your circuit boards, make sure to order the lead free finish. It's better for the environment, better for you [ lead can cause all sorts of bad stuff to your health, and specially to children ], and looks far better than the leaded variety.
One thing to have in mind for future orders, is that most manufacturer give special attention to large volumes of boards, mostly by price [ they set some pretty steep base prices for most production services ], so if you only need few boards, always make sure they have standard orders [ most likely small boards, 3 to 5 boards is common ]. Also make sure to consider all the options, because sometimes a little extra on a board can get you a long way on quality and looks.
Now that you have ordered your circuit boards, check each and every one as soon as you can get your hands on them. Should you find a defect such as broken traces, broken or chipped edges or anything like that, its better to have that checked before you start assembling your project so you can get a quick replacement without any hassle.
So there you go! If your boards arrived safe and sound, your project should be well on its way to be a professional looking piece of technology.