My jigs are never things of great beauty, what I want is functionality and a quick make.
The wood is ash and is cut and planed square. It does not matter what size wood is used, just make it with what you have and to the size you most commonly use. This is a big one and could have easily been made smaller.
A 45 degree square was used to mark the line from the bottom left corner. Using a tenon saw I cut about half an inch into the wood. The metal marking strip is a bit of bandsaw blade with the teeth left on. This was easily enough banged into the sawn slot.
Not having ever banged a toothed bandsaw blade into a slot before I thought it best to leave the edge square instead of sharpening it first. I finally used a carbide sharpener I was given years ago. I still do not like it, bloody useless tool. Do any of you use one? Do tell, and do you find it useful?
So onward to using a file. This was all taking too long for my liking so I banged the blade out and sharpened it on a linisher. This is what I would advise you to do, and bang it in with a block of waste wood between it and the hammer. The bevel is about 80 degrees, obtuse and durable.
Screw the guides in making sure that they are exactly 90 degrees to each other.
To use place the wood in the corner, do make sure it is a good fit if you want accuracy. A gentle tap will mark a diagonal line corner to corner. Turn the wood 90 degrees and bang again. The intersecting lines will be the centre of the wood.
I turn a lot of smallish beads. I make longer billets of wood, when one end has been turned with six or so beads then the wood whips to much. I cut the beads off and start turning the much shorter other end. This jig will mark cylinders as well.
The square edge of kiln dried oak can be sharp and does fray the lathe cord, so I make up a simple sleeve from MDF and Gaffa Tape to protect the cord and also the square edge from being damaged.