Truing a bicycle wheel can be a time-consuming and frustrating experience. A truing stand can make the process much easier, giving feedback as to where the wheel is out of true and which spokes to tighten or loosen. A truing stand can cost over £130, so it can be cost-effective to build your own.
Widen the fork by bending blades away from each other. This will allow the fork to accommodate rear wheels, which are wider than what the fork was made to hold. Take care to bend each fork blade the same amount.
Measure and drill one hole in each fork blade to fit the screws you have. These should be 12 1/8 inches from the centre of the fork dropouts in order to fit the "calipers" for truing 700c wheels.
Measure and drill another hole in each fork blade. These will be to fit 26" mountain bike tires, and should be drilled 10 7/8 inches from the centre of the dropouts.
Install the "calipers". For each hole you drilled in the fork, install a nut onto a screw, turn it about halfway down, then put the screw into the hole in the fork so that the point aims toward the centre of the fork. Next, put another nut on the screw and tighten it to the fork blade. These screws will be the adjustable "calipers" that indicate which part of your rim is most out of true, and the tips should just barely touch the centre of the rim for whatever wheel you wish to true.
Mount the fork in the vice. This provides a stable base for the truing stand, so that you can make precise adjustments to your spoke tensions.