Simply putting a ruler or tape measure into the guitar and feeling for the heel block works perfectly well. Looking into the sound hole to see the measurement at the apex of the cross brace works well too if you have adequate light. Facing the tape or ruler towards the back of the guitar and using a small mirror and a light, (phone light, flashlight etc.) will also work. If you are having trouble seeing the measurement precisely then erring on the side of caution is a wise move. It is very easy to cut some more off the alloy if the unit is too long. You have to subtract 3/16 of an inch, (5 mm), from this distance and this is the length you cut the alloy to. Measure the alloy from the tip of the angled end and mark and cut the square end of the alloy, not the angled end.

The hole that you drill should be centred behind the D and G bridge pins and allow for the 8 mm diameter cover dot that will cover the machine screw head after installation. Very occasionally some bridges do not have adequate space to allow for a countersunk hole behind the bridge pins in which case the 12 string version of BridgeMagic© can be used as it requires that two holes be drilled either side of the bridge pins, (refer to installation pictures). On the 12 string version be aware of the position of the top braces inside the guitar so as not to drill into them by mistake. When drilling the hole and positioning the device, be aware of the position of pickup wires, endpin jacks / preamps etc. 

It's important to note that the device does not have to be perfectly positioned in order to thread the screw into the nylon slider. Holding the unit inside the guitar with one hand and turning the screw with the other you can easily feel the screw inside the guitar with the hand that is holding the device and therefore guide the screw into the slider. Once the screw has connected with the slider it is easy to move the device into the correct position before continuing to tighten the screw.

Do this by hand with an appropriate Phillips screwdriver.

DO NOT USE A POWER DRILL

In most cases you can simply tighten the screw until the top is flat or near flat but it is always a good idea to take your time. This allows the guitar top and top bracing time to 'accept' the reversal of the forces that have caused them to distort. In cases of severe bellying (as per the instructions sheet) it is advisable to do the tightening in increments over as long as necessary to bring the top back into shape. This can be done over a few days if required.