As with all Feathers and Wings, you have a choice of headers to solder on. I recommend the Feather Stacking headers for the Feather M0 WiFi itself, the included male header strips for the 7 segment backpack FeatherWing and OLED FeatherWing, and the Feather Header Kit for the Doubler (the Doubler comes with one pair of stacking headers and one pair of normal headers; I prefer the normal headers, so I always order an extra Feather Header Kit with each Doubler I get).
This header arrangement maximizes the visibility of the LED display (and I don't think other headers will fit along with the LED display anyway).
If you are new to soldering, read one of the excellent guides to perfect soldering.
First, solder the stacking headers onto the Feather M0 WiFi. Put the male pins below the board and the female header above the board. Tack down one pin first, then eyeball it to make sure the headers are at 90 degree angles to the board. When you're satisfied with the alignment, solder the rest of the pins. Repeat with the other piece of stacking header.
Second, solder the headers you like onto the Doubler. Be careful to be sure they are at a 90 degree angle to the Doubler. If you want, you can put the Feather into the headers when you solder them in to assist with the alignment. This might make soldering them in straight a little easier.
After you have the Doubler constructed, put the Feather M0 WiFi in it. Make sure the JST connector for the battery is on the outside edge.
Third, solder the male header strips onto the 7 Segment Backpack FeatherWing. The LED outlines should be on the top of the board. You'll need to shorten one of the header strips (I use a pair of needlenose pliers and snap off the header to the correct length) to fit in the 12-pin side of the FeatherWing.
Fourth, carefully insert the pins on the 7 segment display into the FeatherWing. This may require a bit of patience, but work slowly and it will eventually slide right in. You want to avoid bending the pins on the display while doing this. Flip the FeatherWing over and carefully solder the display pins on the bottom side of the board. Be careful not to heat the IC or its pins too much.
Put the 7 segment FeatherWing on the other side of the Doubler.
Fifth, solder the headers you like onto the OLED FeatherWing. I prefer the included male headers for this because I'm never going to stack anything on top of the OLED display. Put the OLED FeatherWing on top of the Feather M0 WiFi.
That's it! That's all there is to the soldering.
Finish assembling the clock. Plug in the battery (if you want to use one; you can power it from USB if you like). I have enough LiPos lying about that I went for the disconnected and mobile approach with the battery.
Next: the Code