LEDs do have polarity. The bases are not perfectly round. There is a flat edge on the negative side of an LED.
The footprint of the LED is silk screened on the board. Not all of the LEDs are arranged with the same polarity. Check every LED for the proper alignment.
This is the focal point of the project, so we want it to look nice. Once the LED is placed in the board, I recommend a small piece of masking tape to hold it flat on the board while it is being soldered in place. If nothing is holding LEDs in place, they may solder in place at unwanted angles.
Once they are soldered in place, the leads can be clipped off.
Placing the LEDs in the four corners first, will permit an even work environment, and make placing LEDs straight and flush against the board, easier.