Arrow shafts (also called the stele) were traditionally made from all sorts of woods - ash, cedar, oak to name but a few. Commercially available shafts these days are usually made from Port Orford Cedar (although again there are others woods available) so all I need to know is what is the poundage of the bow you're shooting from?
Write down the poundage of the bow you are shooting from e.g. 40lb bow.
Arrow shafts come in the commercial length of 32". If you really want to be economical I can cut down the arrow shaft to match your draw length - the draw length is the distance between the front of your bow hand and your anchor point (the place where you know your other hand is at full draw). Over drawing a bow can mean the arrow slips behind the bow which is something you'll want to avoid as it can get trapped. So unless you really need that arrow shortened I'd advise keeping the 32" arrow as standard because too long is better than too short. However if you do want it shortened you'll need to workout your correct draw length and let me know. Get a tailors tape measure, stand in your natural archer's pose with your bow hand trapping the end of that tape against a wall. Pull the tape back like you're drawing the string on the bow but let the tape run through your fingers. When you get to your anchor point (the point where you usually draw the bow back to e.g. your chin, the corner of your mouth, your ear if you want to be really medieval) pull the tape tight and grip it. That distance from the front of your knuckles to your achor point is your draw length. Any problems let me know.
Write down the length of the arrows you require. e.g. 32" (that's the standard)
Now we just need the diameter. Very light bows may benefit from 5/16" thick shaft diameters (roughly 8mm), but most go for 11/32" (closer to 9mm). You're going to score more points at target archery with 11/32" shafts because a line cutter with an 11/32" wouldn't be with a smaller 5/16" arrow. Heavy warbows (100lb+ draw weight) need arrows that are closer to 1/2" thick (nearly 13mm) and you'll need to check out Beyond Step 7 when you're done with steps 1 to 7.
Write down the diameter of the shafts you require. e.g. 11/32"
If you don't want to get any more complex than that move on to Step 3 - Nocks. Still here? Then welcome to one of the most complex parts of the arrow. Below is a video from Pixar's film "Brave". Take note during the end where an arrow is shown accurately being shot in slow motion.... (if the video is too small run your cursor over the picture and click on the hatched box in the bottom right hand corner to make it full screen)
Notice how the arrows appeared to be swimming through the air like a fish? That "bendiness" is measured using the technical term of "spine rating". To cut a long story short if an arrow is too stiff for the bow it will shoot to the left of the target, if it's too bendy for the bow it will shoot to the right, if it's really too bendy it'll flex too much and shatter and possibly injure you and the people arround you. So firstly it's most important to match the correct spine rating to the bow for safety's sake. Then, if you're looking for consistency you'll want arrows with matched "spine rating" or stiffness so you're not compensating for arrows shooting left and right because they're of differing flexibility. A Western European medieval military archer on receipt of a bag of livery arrows (a bag of 24 standard issue arrows which may have been of variable quality) would have pulled each one out, checked for straightness, flexed them and matched arrows together based on this simple test. Evidence suggests that archers would also shoot some arrows, then cut the good one's down to makie "bearing" or "best" arrows which would have been kept in their belts.
If you are putting reproduction medieval arrow heads on an arrow this will also affect the spine rating. How so? Well imagine having a plastic ruler with one end held firmly in your fist. Imagine waving it around and feeling the plastic flex. No imagine adding a big lump of blu-tac or similar to the end and waving it around again. What you'd see is that ruler would be bending and flexing a lot more due to the weight on the end. This is what happens when you put a heavy metal reproduction arrow head on the end of an arrow designed to take a lighter modern field point. Just keep this in mind and contact me if you're worried about arrows breaking when shot.
Okay, let's move on to Step 3 - Nocks