Add more detail to mod README

- Print and assembly tips
- Some images to show what the mod actually is and what to cut off the
  secondary buttons

hardware/Mechanicals-BTU-Mod/README.md

@@ -14,6 +14,8 @@ Designer: [George Bryant](https://github.com/gbrnt) with help from [/u/d4baller]
 ### v3
 This version was designed in Rhino and so isn't really editable for most people. It works well though.
 
+![A trackball with this mod v3 installed](example-v3.jpg)
+
 ### v4
 This version was redesigned in FreeCAD and the various dimensions are now editable in the FreeCAD file.
 
@@ -34,7 +36,26 @@ The particular BTU this mod is designed for is the Bosch-Rexroth R053010810, or
 * [Spareparts](https://www.sp-spareparts.com/en/p/r053010810-bosch-rexroth)
     * Ireland-based? Ships to UK, and maybe elsewhere.
     * Does not require you to be a company.
-* [Rodivago](https://rodavigo.net/es/p/bola-transportadora-con-carter-de-metal-laminado-ref-053010810/557053010810)
+* [Rodivago](https://rodavigo.net/es/p/bola-transportadora-con-carter-de-metal-laminado-ref-053010810/557053010810
     * Spain-based
     * May require being a company.
-    
+
+## Printing
+With a speed of 50-60mm/s, support on the build plate and a layer height of 0.3mm you should get a print time of ~12.5 hours. It may be worth tweaking various settings to reduce it further.
+
+Highly recommended to enable support interface and splitting the support into chunks, as otherwise it can be very hard to remove.
+
+## Assembly
+* Remove all support material from the print.
+* Inspect the holes the BTUs will sit in to make sure there are no lumps inside the hole. File/sand/cut them off if needed.
+* Install the BTUs by pushing them into the holes.
+* Check the BTUs are fully installed by putting the ball on top. It shouldn't scrape on the top surface. If it did one or more of the BTUs might not be fully pushed in.
+* Remove the original top piece from your Ploopy trackball.
+* Remove the secondary button piece, grind off the stop (see below) and reinstall.
+* Install the new top piece on the trackball.
+* Give it a try!
+
+### Modifying secondary buttons
+There is a small tab on the secondary buttons which collides with the closest BTU. It needs to be sanded, filed or cut off. It is indicated in red in the image below:
+
+![Ploopy secondary buttons, with the tab on the end highlighted in red](trim-tab.jpg)