Create a new Photoshop document and set the same dimensions (Width, Height) as I did.
Choose Gradient Tool (bližnjica je tipka G).
Also recommended:
Press D key to set blac/white foreground/background then click the black foreground thumbnail and choose a color you like (see picture).
Choose the Linear Gradient (see picture).
Create a gradient.
Click and hold at the top of the document then drag the mouse the bottom of the document to create a gradient.
In the menu click Filter, Pixelate and choose Mosaic...
Set Cell Size to 40 square and click OK.
In the menu click Filter, Sharpen and choose Sharpen.
Press Ctrl (Command in MacOS) and J at the same time to duplicate the layer.
In the menu click Edit and choose Free Transform.
In the tool bare above set the angle to 45 degrees and confirm by pressing Enter.
Set the layer's blending mode to Soft Light.
Select the original layer (Background).
In the tool bare above set the angle to -45 degrees and confirm by pressing Enter.
Choose the Crop Tool (C for a keyboard shortcut).
Now you want to select the area that is covered by all of your three layers and press Enter.
In the menu click Layer and choose Flatten Image.
Now press Ctrl (Command na MacOS) + F once or twice to make the final result a bit sharper.