The location and dimensions of the slot has to be very precise, so that each channel is the same height as all the others, and placed accurately on the fretboard. You can use the Bear Meadow Fret Calculator to find the fret locations. If you don't know your scale length, measure between the nut and the 7th diatonic fret and multiply by two. (Measuring between nut and saddle will give you an error caused by possible saddle compensation.)
The actual measurement from nut to your new FlexiFret (or from the next lowest diatonic fret) is longer by the distance from the edge of your router base to the center of the cutter (not to the edge of the cutter!). I use the Bishop Cochrane router base shown here, and the distance from the edge of the base to the center of the cutter is very precisely 7/8". So I'll add .8750" to the distance from the existing fret to the center of the new fret. It helps a lot here to have the digital caliper, since you can simply zero the calipers at that offset, then read measurements directly off my fretting chart. So, mount the .090" flat mill in the router and measure the distance from the edge of your router base to its center. This is the distance you will have to add to your fret distance.
The process begins with a very accurate measurement from the nut to where you want your new FlexiFret to be located. This measurement can be made either of two ways. If you have a 24" caliper, you can measure directly from the nut to the desired new FlexiFret (see photo at left). Or you can measure frome from the apex of the crown of a neighboring fret using a 4" or 6" caliper (see photo at right). You need to measure to the nearest .001" -- better yet, if your digital caliper allows, measure to the nearest .0005" or .001mm. Of course, no one could hear a pitch difference arising from such a small measurement error. We just don't want errors if we can avoid them.
If you are worried about how accurate you can be, you can use the Bear Meadow Fret Calculator's Error tool to find out how accurate your measurement can be in terms of cents (100ths of a semitone). Multiply your ruler's smallest increment by two and enter it as the "Error D" (deviation error) in the error column's header (the right-most column). The column will list the deviation from true pitch for each fret with that much error. (Quite a handy feature, if I do say so myself.)
The slot will be cut by clamping a small accurate square to the edge of the fingerboard ) and running a router base across the fingerboard, making multiple small passes until the depth is .09375 (3/32").
The setup for cutting the slot is pretty simple. A small square is clamped onto the fretboard at the centerline of the fret's location (plus the distance from the cutter center to the router base's edge. It is left loose while the distance from the nut (or lower neighboring fret, if you are using shorter calipers) is being set, then clamped tight.
Once you are sure of the placement of the fret and the position of the machinist's square, clamp the square firmly. Reset the caliper without the offset and gently mark the fret's position using the caliper's inner jaw tip. Scrip a line through that point and place a strip of Scotch tape across the fretboard centered on the line. This tape will be used to help find the point where the cutter tool makes contact with the fingerboard. And when your tool has cut through the tape you have a signal that you have gotten just down to the surface.
I cut the slot with a Dremel variable speed Moto-Tool, held in a special high-precision router base available from the guitar builder, Bishop Cochrane. This router base has an excellent yet simple depth stop and a fine depth feed control. The 3/32" slot for the channel is cut into the fretboard with a carbide end mill, .090" (which is then dressed to the final .093" width and thickness with a fine machinist hand file).
To begin, mount the cutter in the chuck firmly, and raise the cutter until it is above the bottom of the router base. Now place the router base on the fingerboard, firmly against the blade of the square you've clamped on to denote the position. Look to see if any neighboring frets are under the router base, causing it to tilt. If so, use some double-sided tape to attach two 1/16" straps or "shoes" to the base, in positions where the base will be stable, and these "shoes" avoid bumping into the frets.
Next, chuck the .090" endmill in the dremel/router. Raise the cutter so it is clear of the bottom of the base. Set the router on the fingerboard, firmly against the blade of the square you've clamped in place. Turn on the router to its slowest speed and gently lower the cutter until you can see it cut into the tape. Continue just a very small distance more until it is just brushing the finish of the fingerboard. This is your zero. Stop the router and set the depth stop at .093" from this depth (you can use the brass channel as a spacer for this, and get a very accurate set).
Now turn on the router and advance the cutter a small amount. Put the router against your clamped-on square, with the cutter clear of the fingerboard, and gently but firmly draw the cutter across the fingerboard. Do not hurry, but don't go too slow either. There will be more accuracy if you just pull the cutter across the fingerboard with a moderate speed. Continue until the cutter is well clear of the other side of the fingerboard. Advance the depth a small amount and take another cut as before. Continue taking small cuts until you reach the depth stop. Brush away any cuttings and take the final cut again, just to make sure the router base wasn't resting on sawdust (and thus cutting the slot too shallow). Set the router aside
Now remove the tape and inspect the slot. Here's a closeup of channels after the installation is complete. Try a brass channel in the slot. If it is too tight, lightly file each side of the slot with the fine file. The file should have one edge that doesn't cut. Keep that edge on the bottom, for now. (If you file has teeth on both edges, you can make one edge "safe" or non-cutting by dulling it on a whetstone. Make sure you keep the bottom square with the sides if you have to do this.) Make small adjusting cuts (Be careful to file straight across the cut. Any sidewise rocking will create a slot that's narrower in the middle and than at either end.) until the channel slips into the slot with no binding whatsover, but also without any wobbling -- just a nice smooth fit.
Now look at the depth of the slot. With the channel sticking out half way out of the fingerboard, put a small straight-edge, ruler, or square firmly on the fingerboard, over the channel. Move channel in and out of the slot a little, noting whether the channel drags the straighte-edge with it -- an indication that it is too high. Turn the cutting edge of the fine file down to file away the bottom of the slot. (Again, be careful not to rock the file across the cut, creating a slot that's high in the middle and low at either end.) Make fine cuts, testing often. And make sure you test for depth with the straight-edge all across the fingerboard.
Now we are ready to install the channel. For that, we will install the channel.