It's very simple.
First you have to get a wide collection of atoms between 2 mirrors. Then you can excite the atoms using heat. This excitement will emit photons from each atom. These photons will then move back and forth between the mirrors. And as you excite more atoms , more photons will be emitted. You can then make a hole in one of the mirrors causing a nice light beam beam coming out of it called a laser.
Click on this link for a graphic picture of the laser