How can I make this effect?

Just looks like a halftone overlay on some block color image.

This is either screen printing or supposed to replicate it.

Screen printing works in layers, you have a red background/paper, then a layer of green that creates the landscape and finally a black layer for the soldier.

You usually want to go from the background forward: 1 to cover mistakes and 2 because it makes sense that the closest thing is on top of the farthest thing.

With that being said, Google "screen printing, halftone effect in Photoshop"