Plugins Needed:Sepia 2 (Optional)
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Don't know how to install plugins?
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Before we start, here is the outcome:

Thanks to the old Paint.NET user
indie88 for some tips on making this!
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Before we start, decide what size you are going to use. I chose 800x600.
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Now, run
Effects -> Render -> Clouds. Use the default values:

Now, add a new layer, (
Layers -> Add New Layer) and run clouds again.

On you new layer, set the blend mode to "
Difference."

Now, add 5 layers with clouds and their blend modes set to "
Difference." (Do not duplicate)
After you do that, it should look like this:

Now, flatten you image. (CTRL+F) You should have only one layer. It should be called "Background."

Now, duplicate this "Background" layer twice.

Emboss (Effects -> Stylize -> Emboss) your first duplication at -45 Degrees.

Now, go to Adjustments -> Brightness / Contrast and lower the brightness all the way down.

Now, set this layer's blend mode to "Overlay." (Press F4)

You have probably noticed that after you do the previous 3 steps, nothing has happened. But, all of these effects and steps you have just done will come into play in the next steps.
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Now, emboss (Effects -> Stylize -> Emboss) your second duplication at 135 degrees.

After you do that, your image should look like this:

Now, set this layer's brightness all the way up. (Adjustments -> Brightness / Contrast)

Your image should now look like this:

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Okay, you could just end here, but I like using this step at the end of my texture. First, flatten you image. (Image -> Flatten) Now, go to Adjustments -> Sepia 2. Run it at 00.30.

After you do that, repeat using the same settings.

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You are now done. Feel free to add any other effects you want.