In this step, you're going to apply solder to one of the microcontroller pads to help with alignment when you solder it to the board.

Press the iron's tip against the board's bottom left microcontroller solder pad for a few seconds to heat it.

If you heat the circuit board continuously for more than 10 seconds, the excessive heat may damage the solder pad.

Feed the solder wire into the heated area until there's a small glob of solder on the pad.

Since these pads are so close together, it's easy for the solder to adhere to multiple pads. This is fine as long as the pads aren't bridged to each other.