You will need to create a custom shadow. To do this, ensure your “fold” is selected, hit Ctrl-J to duplicate the layer. Hit D to return the color chips to their default setting and with fill the “fold” with black, but using alt + backspace. Make sure there is an outline selection around it, or it will fill the entire layer with black, you only want the triangle filled.

Move the “shadow” below the original “fold”. If it’s easier for you, you can always rename your layers to keep track of them. With the shadow layer highlighted in the layers palette, go to filter>blur>Gaussian blur. Adjust the slider anywhere between 2 and 3. Lower the opacity of the shadow layer to around 50%. I set mine at 48% because that is what looks good to me.

Select the smudge tool . We are going to use this to pull the middle edges of the shadow outward and pull the three points of the shadow inward. You need to play around with the size of the brush until you get it where you need it. I set my brush to 90 pixels and a strength of 50%. The image above gives you an idea of what you will be doing with the smudge tool. To get the desired effect you will need to practice. I have only pulled out the middle portions so far.

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You can stop here if you like the desired effect, but if you want to add more realism, you will need to add a highlight to the fold itself. To do this, you will need to use the dodge tool.

I normally set it at midtones and then somewhere around 10-20%. I have the size set to 40, but once again that is something you will need to adjust depending on the size of your fold. Then you want to brush across the top of the fold to make the highlight. 

Okay, now I have my highlight on the top of my fold. Now using the burn tool, I will do something similar but I will hit the tip of the fold and the back edge of the fold.