God of Light: Remastered

There are a total of 150 fireflies to collect, one in every level. To tell if you've collected the firefly in a level, from the level select screen there will be a big light blue glowing icon rotating over the level number if there is still a firefly to find in that level. 

As far as collecting fireflies, you'll be shown how to do this in the tutorial: direct a light beam at it until it grows and bursts, at which point 2 more seconds of time will be added to your firefly counter in the upper right. As noted in the roadmap, I recommend finding the firefly on your first attempt of each level while you set it up to solve it with three stars. The firefly is typically sitting on a rock somewhere just out of view, so each time you're in control of a new beam of light, move it to each extreme to look for the firefly. Once you find it and point a light at it, it will start to glow and make a noise. Then just direct the beam at it until it pops. Do this with the single firefly on every level to unlock the achievement. 

You can alternatively just play through each chapter, and then from the chapter select view you can see which levels you missed the firefly on (you will end up collecting some inadvertently anyway) and just replay those levels to get each firefly. If you do this, you don't have to complete the level again. As soon as you collect the firefly and the time is added in the upper right, you can pause and quit out to the main menu (level select) and it will save automatically.