First off I have seen and heard many people say just get a orbital valve and a ram, plumb it up, and go have fun. A lot of times this will work but I decided I would do a write up on how to do it correctly.
Make sure you read everything before deciding on any part choices!
Components:
Pump (if using a factory pump most have reservoirs and pressure relief valves in but still check your pump to make sure)
Reservoir
Pressure Relief Valve
Orbital Valve (they are also called steering control valves or metering valves)
Hydraulic Cylinder
Cooler (Trans coolers work great)
First you want to decide on what pump you are going to use. Once decided you need to figure out what the flow rate of the pump is (most are between 3-5 gallons per minute). The higher the flow rate the faster you will be able to turn. Also check to see if your pump has a reservoir and a pressure relief valve.
Next you have to pick an orbital valve. There are many, many different ones out there. A common misconception about hydro steering is that you cannot turn if your engine dies. If you get the correct kind of orbital valve you still can it will just be hard. Basically they have check balls inside that block the return of fluid to the reservoir if the pump stops pumping. This basically turns your orbital valve into the pump and when you turn the wheel you manually force the fluid into the ram. Once you have decided on the valve you need to get cubic inch displacement of the valve. This is how much fluid it displaces in one 360° turn of the steering wheel.
Next you need to get your ram. How to decide on a ram all depends on your pump's flow rate, your orbital valve's displacement, and how many turns of the wheel you want to have to turn your tires lock to lock. If you valve displaces 5 cubic inches per 360° rotation and you want 3 full turns of the steering wheel to go lock to lock you would need a hydraulic cylinder with volume of 15 cubic inches. The formula is this: (orbital valve displacement) x (desired number steering wheel turns lock to lock) = (required volume of hydraulic cylinder).
Last you need to calculate the entire volume of the whole system and make sure you pump will put out enough volume for the system. Remember higher pump volume output or flow rate will give you faster steering. For example if your pump output is 3 gallons per minute and entire system volume is 2 gallons with the hydraulic cylinder completely compressed in and 5 gallons with the cylinder completely extended it will take 1 minute for your pump to put enough fluid out to completely extend your ram (obviously these numbers are not real world numbers I just used them to get the concept across). Also remember your pumps flow rate changes with engine RPM, the higher the RPM the more volume your pump puts out. Check out this other post I made about modifying a stock pump for hydraulic steering: http://wegotmud.com/forum/index.php?topic=70504.0 (http://wegotmud.com/forum/index.php?topic=70504.0)
Also you can use an electric pump. This will give you consistent volume at any RPM and allow you to have full hydro steering if your motor dies (until your battery dies).
I feel these are all the components I need to go into in depth. Here is a diagram to show you how to put it all together:
(http://i775.photobucket.com/albums/yy34/bmx0964/Tech/hydraulic_steering_diagram.jpg)