I recommend using a PCB building service from JLCPCB and similar. 
They also supply the components for an extra fee. 

Building it yourselve can be challenging, since the board is designed to be very small to fit in the space.
But if you want to try go ahead! You can use a hot plate like (Miniware MHP30) it worked well for me during my troubleshooting and fixes before ordering new boards.

The project is designed to be just 1 sided, like Watchy, this allows a flat placement of the eInk display on the back, providing extra support and allows easy soldering and mounting.

The case is printed in 2 separate pieces front and rear.
I used again an online high quality service, otherwise the result might be rough at home.
But i printed some at home and they were quite good (except for the border of the solar cell).

I used glue (loctite or similar) to glue the pieces together as well as the components to the case.

After closing the unit, i live with it, or break it apart to get the components back.

The solar cell comes per-soldered with some cables, these interfere with the panel.
It is better to remove the glue and unsolder these cables leaving the solar panel ready to solder again.

First, place the solar cell in the front case check that it fits ok, and check the positive/negative of the solar cell. Apply some glue to the border of the case where the cell goes (apply VERY little quantity). Place the cell and clean extra glue around the cell.

Then, place the eink display on top, the cable of the eink should point to the solar cell. The cell will be free to move up/down a little. Ensure you push the eink display as far as you can to the solar panel side, leaving space at the bottom for the LED installation (you can check the pictures I uploaded of how I did it).
When you are ready, place some glue in the sides where the eink rests, and place the eink display.

Then continue with the LED, you need to place the LED under the display, but pointing up towards the display. This is so the light enters the glass and reflects internally in the display illuminating it. You can use super glue again.

Finally, put the board in, connect the display first, then lay it on the display back.

All the cables you can leave them there, no need to solder them yet.

I used stainless steel bolts to make the buttons. It is easy just screw them in, however the issue was how to properly connect the cables to them. How I did it was to pass the cable trough the hole (striped from plastic already), then screw in with the cable in, this way the cable was surely touching the screw. Then I trim the extra cable coming out, and the small bit i bend it under the screw.

I am welcome to alternatives, maybe tin screws can be a better idea since they can be soldered to the cable.

The V4 hardware right now has a broken antenna and will not work properly, also GPS occupies a lot of space so it is definitely hard to install these 2 parts, however, if you want to do it anyway:
I recommend desoldering the SX1262 antenna port, before soldering the component to the PCB, after that solder a cable, and at the end soldering an external coil antenna.  (I know, impedance! How dare you!, but it works, 3/3 it works, the SX1262 is quite insensible to these things, and for short ranges it works).

Then, wrap the antenna if teflon foil to avoid short circuits.

For the GPS is quite straigforward, but the Goku Nano 3 in the slot, and solder it. The connector is not very useful anyway so i desolder it and solder using the pads.

The Touch connections needs to be soldered ot the pads, same for the Piezo speaker and vibration motor.

Wrap these components in teflon foil to avoid short circuits. Should look like this:

Finally solder the batery and solar to the big pads at the bottom, before doing that you can test things work with the USB cable.

Once everything is done you can put the rear case in and check if it closes, order the components so it does close.

I recomend adding some glue for the vibration motor, otherwise it will shake loose in the case.

When you are ready add super glue and some weight so it closes properly.

For wristband you can use any commercial one 22mm.
For you can also use a USB magnetic accessory for style, and to block the USB port from getting dust or dirt (or simply for easier plug unplug for firmware changes)