I will put the tips and warnings below.

If you you don't know what it is, it is like pre-chewed gum that can stick things up temporarily.

It is highly recommended to test the microcontrollers and modules before soldering them together. Since I use the stamp soldering technique, it becomes extremely difficult to replace a part once it's soldered. I learned this the hard way when I discovered the microcontroller was defective after soldering everything together. Jumper wires with hooks at the ends are very helpful for testing.

The order is to solder the pins first and then the screw holes.

It is suggested to leave a small space between the module and the PCB, so you can use a flashlight to check if they are actually soldered together. While you can always use a multimeter, a visual check is also helpful.

I used tweezers to touch the bottom of the pin hole to ensure the connection.

The hardest part is the GND pin, which has the largest connection and is therefore the hardest to heat up. I don't have a better solution than using a higher temperature on the iron, more solder, and more heating time.

The order should be to stamp the module by soldering all the pins first and then solder the battery pins through the via. (I tried the reverse and destroyed the battery pin pad.)

The battery pin pad is a bit hard to reach. The trick is to use a male pin to touch the bottom of the via. Using a sharp tool such as tweezers might also work, but a male pin attracts liquid solder and thus works better.

When soldering the battery pin, connect a battery and a USB power source. The charging indicator on the Xiao can be used to detect if they are properly connected.