The ESP8266 development board is a good go-to board for your IOT projects, but presents problems if they are battery powered. It is well documented how the various ESP8266 development boards are not power efficient (here and here). The Witty Development Board overcomes some of the problems by having a separate USB to TTL (programmer interface) but does not have the same shield support of the D1 Mini.This D1M BLOCK separates the ESP12 from the D1 Mini so that it can be used to program ESP12 modules (bare or with efficient regulators).

Step 1: Materials and Tools

There is a full Bill of Materials and Sources list.

The Wemos D1 Mini Wifi board with ESP12 module
3D printed casing and labels.
A set of D1M BLOCK - Install Jigs
Strong Cyanoachrylate Adhesive (preferably brush on)
Hot glue gun and hot glue sticks
Solder and Iron

Step 2: Removing the ESP12

The ESP12 module needs to be removed from the board with both pieces remaining usable. Using a hot air gun is my preferred method but there are others.

Remove the ESP12 chip using one of the various methods with a clean undamaged separation between the two parts.

Step 3: Soldering the Header Pins (using the PIN JIG)

There is a video above that runs through the solder process for the PIN JIG.

Feed the header pins through bottom of the board (TX right-left) and into the solder jig.
Press the pins down onto a hard flat surface.
Press the board down firmly onto the jig.Solder the 4 corner pins.
Reheat and re position board/pins if needed (board or pins not aligned or plumb).
Solder the rest of the pins

Step 4: Gluing the Component to the Base

Not covered in the video, but recommended: put a large dob of hot glue in the empty base before quickly inserting board and aligning - this will create compression keys on either side of the board. Please do a dry run in placing the shields in the base. If the gluing was not very accurate, you may need to do some light filing of the edge of the PCB.

With the base casing bottom surface pointing down, place the soldered assembly plastic header through the holes in the base; the (TX pin will be on side with the central groove).
Place the hot glue jig under the base with the plastic headers placed through its grooves.
Sit the hot glue jig on a firm flat surface and carefully push the PCB down until the plastic headers hit the surface; this should have the pins positioned correctly.
When using the hot glue keep it away from the header pins and at least 2mm from where the lid will be positioned.
Apply glue to all 4 corners of the PCB ensuring contact with the base walls; allow seepage to both sides of the PCB if possible.

Step 5: Gluing the Lid to the Base

Ensure the pins are free of glue and the top 2mm of the base is free of hot glue.
Pre-fit the lid (dry run) making sure no print artifacts are in the way.
Take appropriate precautions when using the Cyanoachrylate adhesive.
Apply Cyanoachrylate to the bottom corners of the lid ensuring coverage of the adjacent ridge.
Quickly fit the lid to the base; clamping shut the corners if possible.
After the lid is dry manually bend each pin so it is central in the void if necessary.

Step 6: Adding the Adhesive Labels

Apply pinout label on underside of base, with RST pin on side with groove.
Apply identifier label on flat non-grooved side, with the pins void being the top of the label.
Press labels down firmly, with a flat tool if needed.