Step 1: Choose Your Photo
After you've settled on a photo, open it in Photoshop and do any basic image/spot correction that you want. This photo is too dark, so I am going to open up Image>Adjustments>Levels to lighten it.
Even if your photo has good lighting, open up Levels and play with the inputs a little bit. We want the photo to start looking more like a drawing with less realistic color. If you are using a drawing, you may skip this step if you feel it necessary. For my photos, I usually bump up the gray and white levels, but each photo you use will have a different result. Keep playing around until you find the right balance. If you go too far off, hit cancel and start over.
Depending on the photo or drawing, you might want to add a little bit of sepia color to give the photo more of an aged look. I am going to use a fill layer in this case. Layers>New Fill Layer>Solid Color. Change the Mode to Soft Light and click okay. Choose a fill color that looks best to you. Merge that layer down onto your photo after you are satisfied with the effect. Sometimes you can play with Levels enough that a sepia layer is unnecessary.
For photos with a lot of detail, you will want to add a Filter>Blur>Gaussian Blur to the photo.