In order to keep a low profile on this wearable, we'll connect the Feather & OLED Featherwing using short headers. Using these will reduce the pin's height by about 3mm vs normal headers, so the boards will lie flatter against a shirt or lapel. That being said, you can use normal headers instead – the pin will just be a bit more awkward to wear.
Insert the male header pins into a spare breadboard in order to keep them stable and straight while soldering.
Place the OLED Featherwing onto the pins.
Solder each pin to it's surrounding pad on the top side of the Featherwing.
Remove the OLED Featherwing from the breadboard and flip the Featherwing upside down. Temporarily mount the female header pins on the Featherwing to keep them in place while soldering to the Feather.
Flip the Feather upside down and place it onto the short pins of the female header.
Solder the short pins of the female header to the pads on the Feather.
Disconnect the Feather from the Featherwing and place the Lipoly battery lengthwise between the Feather's female headers with the JST connector hanging over the Feather's USB jack.
Reconnect the OLED Featherwing to the Feather while being careful not to pinch any part of the battery in the process. Lipoly batteries are somewhat delicate and can be dangerous if punctured!
Connect the battery's JST connector to the Feather's power jack. You can now charge the battery by connecting the Feather to a USB power supply
Peel the protective layer off of the pin back's adhesive side and attach it to the back of the Feather. Placing the pin back as seen above will help prevent the pin from being top heavy and tipping forward while in use. Press the pin back firmly in place to ensure it's secure.