For this tutorial, you will need some simple vector images to use as cut-out. We used one with the outline of “Waves” and other with some “Koi Fish”.
1 - In Illustrator, create a “New Document”.
2 - Select the “Rectangle Tool” (M), draw a rectangle that covers all your document and fill it with the color of your choice (we used #00001d).
3 - Drag your Layer to the small “Create New Layer” button at the bottom of the “Layers Panel” to “Duplicate” it. Lock your bottom Layer by clicking on the left side of its thumbnail in the “Layers Panel” and bringing out the small padlock icon.
4 - Open one of the “Waves” vector file, “Copy” (Command/Control + C) the portions you want to use and “Paste” them (Command/Control + V) into your document.
5 - You may need to Re-size, arrange and/or Rotate or Reflect them to get the results you want. Change the “Fill” color of your second Layer (we used #05aa34).
6 - Select all your “Wave” pieces and “Group” them together (Command/Control + G). Then, make a “Copy”, “Paste it in Place” Shift + Command/Control + V) in another Layer and hide it by clicking on the small eye icon left of its thumbnail for now.
7 - Select the “Pen Tool” § to draw a shape in the center of the “Artwork”. Select it together with the second layer of color, then go to the “Pathfinder Panel” and choose “Minus Front” to punch a hole in the Layer.
8 - Duplicate that layer (the one with the hole in the middle), select it together with the one with the “Waves” and repeat “Minus Front” in the “Pathfinder Panel”. Change the “Fill” color (we used one from the Color Swatches).
9 - With the resulting “Shape” still selected, go to the little menu at the top-left of the “Layers Panel” and choose “Collect in a New Layer” to create a Layer with it. You can re-name the Layers if you want.
10 - Turn back on the copy layer of the “Waves” and give it a “Fill” color (we used #0070ab).
11 - Select all your layers (except the one at the very bottom), in the “Appearance Panel” click on the “Effects” menu, navigate to Stylize > Drop Shadow and use the following values:
We noticed that the “Background” color was too dark so we unlocked the bottom layer and changed to a slightly lighter shade (#1934a5).
12 - With the “Direct Selection Tool” (A) tweak the edges of the shape with the cut-out waves. Then do the same with the layer that has only the waves so they are less uniform.
13 - Create a “New Layer” and paste your “Koi Fish” into it, add some “Fill” colors to the fish and apply the same “Drop Shadow” Effect to them.
14 - Your “Artwork” should look similar to this.