Because your crosshair in the game is white and I had some problems to find it sometimes I highly recommend you to use your background or something black like maybe your steam profile in order to keep sight of your crosshair.

The game has no tutorial but you probably figured out that you can move with "W,A,S,D" and shoot with the left mouse button.
However there is a second weapon bound to the right mouse button. It's a shotgun-like weapon. It's very effective against hordes of low life enemies, so use it for that.

When you are finished with the game, you can replay level 4 a couple of times. This level has only low life enemies and more than 500 of them. You can use your right click weapon to finish them faster.

The picture of the CPU was made by: Eric Gaba – Wikimedia Commons user: Sting.

This was my first guide ever. Because there was no guide, I decided to make one. I hope you enjoyed it and it will help you.