Placing the guides is the most trickiest operation, they must be completely perpendicular respect to the base, and they must be completely parallel to each other to avoid getting stuck.
First I cut two 25Cm long boards (per 10mm wide) (Pic 1), notice I'm not throwing away any wood (Pic 2). After sanding the edges place my set square at one edge, and I do two marks, one at 10cm, and one at 90 (pic 3), I repeat the operation with the lower part of the board and with the other board.
TIP: To do the markings with the set square, use left edge of the small boards (pic 2) this edge comes perfectly straight, so errors and other problems when marking won't be a problem.
Once finished, these lines must be parallel between each other, now, with the set square I do a line 1.5 cm above the lowest edge, perpendicular to the other two, that line will help the guides to be at the same height.
With the lines that define the position of the guides I place them so the longer lines pass through the middle of the holes, then, with the pencil I mark the shape of those holes, and I drill them out of the wood (Pic 4).
TIP: Maybe this is useful or maybe I'm too paranoid, but I marked the guides to know which goes where after marking the holes to drill, theoretically they're exact, but anyway, let's not risk it... After the holes are have been drilled, screw the guides to the small board (pic 5).
Important: Notice the my guides have different sets of holes, on one side there are lots of them, but on the other one there are just a few. The side with just a few holes will be the side attached to the arm, this is because it'll be easier to disassemble in case the guides get stuck, or to do any adjustment...
To screw the board to the spacers we need to follow a special procedure, this board must be perfectly parallel respect the big board, to do so I made a simple but effective tool to align the board (pic 6), I slide it along the flat edge of the board and check that the screw is always touching the small board, once I'm sure the board is aligned I clamp it in place, drill the holes, and without removing the clamp I insert the screws (pic 7 & 8).
After this we just have to screw the guides in place (Pic 9). I
IMPORTANT: While placing the screws the board can change of position to accommodate to the screws, to avoid this place the screws with the clamp attached, that way the screws will have to get accommodated to the wood, and not the other way around.