Reddit mentions of Ricoh GV-2 External Mini Viewfinder

Snap. Focus. This will change the way you shoot more than anything and is much more intuitive than standard zone focusing techniques. Outdoors, I usually set it to 1.5m which makes it hyperfocal at f/9 and close enough to get most everything if you go down a few stops. Indoors, I tend to go with Spot AF because wider apertures require narrower DoF making Snap less useful. Set the AEL/AFL button to lock focus and display the current focal area distance range.

Customize your interface. Every single button on the GR can be remapped, so take full advantage of this to match your shooting style. I like shutter on the front wheel, aperture on the back lever and ISO first on the list to adjust when it is depressed. Fn-1 enables Snap Focus and Fn-2 adjusts Snap Distance. I don't assign the Effect button because I like to have everything accessible with one hand, but it can be set to any function also.

I usually shoot in manual with ISO at or below 1600 if possible, although the noise at 3200 can sometimes be tolerable. It seems to be more even and less colorful than the noise from Canon's APS-C sensors. TAv is an interesting mode that is somewhat unique and lets you fix both shutter and aperture. I don't use it much though because it ramps up the ISO aggressively to levels that are unusable if you're in moderately low light.

I tend to keep ISO and aperture pretty constant and mostly change shutter speed, usually between 200-500. A useful hidden feature while in manual mode allows you to automatically adjust the shutter for current light conditions by pressing the +/- button once. You can also set it to adjust aperture or both (One-Press M Mode). This is super helpful because you can stay in manual without missing unexpected shots in changing light conditions that would normally require time spent changing modes or making significant adjustments.

Stealth mode: turn off all LEDs and sounds, paint or tape over front logo and light, Display off (settings will still show briefly when you adjust anything), OVF if you want to compose (I like the GV-2, though no optical finder is super precise and I don't think it's a must-have)

Batteries: you can never have enough. Probably the only significant limitation of the GR, but fortunately they're small and charge quickly.