I am a designer by trade so I created this card in Photoshop. However because not everyone has access to Photoshop, I decided to put together a tutorial on how to make this Warhol effect in Pixlr Editor, a free web app that is pretty awesome. If you are a Photoshop user, most of the steps are pretty similar, so this tutorial is still relevant. For any experienced Pixlr users out there, tips and tricks on techniques I may have missed are welcome.
Step 1: Setup Image Document
Make a new image (file/new image) and set it to 900x900 (this is 3" at 300dpi, which is ideal print quality). I intended to have a 7" square card with four images, so this was as large as I needed. Make yours whatever size you need, at 300px per inch.
Copy and paste the image you had opened into your new square document.
Resize it to fit the space (edit/free transform). It's OK to size up a fair amount. When using free transform, hold shift to keep your aspect ratio the same. You can resize by dragging from the corner, and if you move your cursor slightly off the corner you will get your rotate option. I resized then rotated slightly.
Step 2: Make Your Image B&W
First, duplicate your color image. Desaturate image (Adjustment/Desaturate).
Now it's time to change your image from grayscale to only black and white. When doing this to images, you may find that different parts of the image need different levels of adjustment. For example because my hair is dark, bumping up the contrast in a way that makes my hair maintain detail will render my face solid white. Because I need a different level of brightness applied in different areas, I'm going to duplicate my grayscale image twice more, and will adjust one copy for the dark parts of the image, one for the light.
On the first copy of the grayscale image, I'm going to go to Adjustment/Brightness & Contrast, push the contrast all the way up, and adjust the brightness until my hair still has some white details in it. To get a truly black and white image you will need to do this step several times. This will totally blow out my face as you can see, which I'll optimize in my other layer. If you are using Photoshop this step is much quicker, as you can check the box "use legacy" to achieve a solid black and white image right away.
In my second copy, I'm going to optimize the effect for facial features. In this version, you will see that my hair is completely black.
Step 3: Combine Layers
Drag your darker layer on top of your lighter layer. While on your darker layer, go to Layer/Add Layer Mask. You will see that your layer is no longer visible. We just masked the whole image. Now we want to add back in just facial features, which are exposed better on this layer. To do this, select the brush tool, and with it set to black, click on the clipping mask icon (white square next to your layer). Start removing the mask in the facial area. You will see green color show up in the areas that will be added back. If you paint too much, use the eraser to cover back up what you have revealed. This is the beauty of masks, with the brush tool set to black and the eraser you can continually adjust what you want to hide or show.
Step 4: Clean Up Background
First I'm going to clone in the gap in my hair on the left side of my head, where there was another person before. I select the clone tool (looks like a stamp), check "sample all layers" at the top, and hold command to select my clone reference. If you are not familiar with the clone stamp, it allows you to duplicate the area that you reference in another part of the image. It is a really amazing tool. I roughly filled in the area where my hair was missing, knowing I'd clean up the background in the next step.
Now color over the black areas of the background in white, to isolate the face against a plain background. Use a brush with sharp edges (not blurry, select at top). If you feel more comfortable, you can create a new layer for this step.
Step 5: Coloring Time!
Create a new layer. Click on the icon on the lower left of the layer panel, to open the layer settings. Set the mode to multiply, which will combine the colored layer over the black and white layers below it.
With a hard edged brush, go color crazy! For a true Andy Warhol look, your colors should be bold and edges between colors clear.
Step 6: Compose Card
For the final step, you will create a new document at your desired size (in my case 7" square, or 2100x2100). Copy and paste your square into your new full sized document. This is the point where I found myself missing Photoshop, as from what I can find there are no alignment or ruler/guide tools in Pixlr, nor is there the ability to press shift while dragging to keep objects moving straight on the x or y. So, wing it! Move things until they look aligned, and call it a day. For Photoshop users, the ability to do this step in a more exact way is easily possible with the align tools at the top (select the layers in question first, then use appropriate alignment tool).
Print your image, attach it to a card, and give your friends and family a personalized card they will love!