Make sure to install the frontpanel with the spacers BEFORE you solder the switches and the leds in place, so they align nice and flush with the front panel.
Now it time to 15 double check all the connections and make sure there are no shorts between Vcc and ground.
Once everything is soldered correctly it's time for the first tests with usb power and share common ground with the modular, a breadboard comes handy to make that connection if you have no spare bus cable lying around or a female/male patch-cable.
This is the time to bug fix the arduino code a couple of times and make sure not to brick its bootloader again. Believe me it's a pain in the neck to unbrick, I posted a thread here with a link to a how-to.
Once everything seems to work fine, it is time to put the fully assembled module in the modular system, for some extra testing.
And now I have to think about what to put on the front panel, and how to make it look pretty.
And that's how you build a 3 track Euclidian (plus one weighed random latch track), trigger sequencer in one unit module. Each track has two outputs, the euclidian tracks are also triggering the leds.
It is definitely an excellent platform for making more complex modules for the AEmodular. I would advice to first hone your coding skills on an arduino uno or something similar; there is less change of messing up your bootloader with buggy code than when coding for the arduino pro micro. The micro however has a small enough footprint to be used in 1U modules. I will also look into teensy modules since they seem to be a bit more powerful/advanced on the interface level than arduinos.