Also note that the strength of the selection depends on the color you use to paint it with. Black is 100% selected, white is 0% selected, medium gray is 50% selected. For this tutorial, use black color to paint the selection.

Now let's start painting that selection! Make sure you're in Quick mask mode and are using the paintbrush with black color.

Here's my selection (in Quick mask mode):

The color and strength of the selected area in Quick mask mode can be set by holding down CTRL and double-clicking on the Quick Mask mode icon in the tools palette. In this picture I used a pure blue color and 75% opacity to make my selections easily discernable. I chose blue color because the picture had no blue color in it.

I drew the selection by hand with the mouse, alternating between adding to the selection with black color and erasing with the white color. (Press X to switch which color to use)

It is a very good idea to save the selection. If you happen to deselect it and don't notice until too late, you might not be able to get the original selection back. Or maybe you need to finish the picture later? The selection will be gone the next time you open the file in Photoshop.
CTRL-clicking the layer to create a new selection from the pixels in the layer won't neccessarily work, since we're going to edit the transparency of the layer. A new selection created that way will ignore pixels below a certain transparency threshold.

So, it's better to save the selection as a new alpha channel. Do it from the menu "Select" -> "Save selection". Give it an appropriate name. I named mine "Slime 1". Now when you check the channels in the Layers palette, there should be a new alpha channel named Slime 1 in there. To get your selection back, just CTRL-click on the Slime 1 channel.

IMPORTANT! The selections you save as alpha channels will ONLY remain if you save your file in a format that supports multiple alpha channels. Preferably save your image in photoshop-format (or your paint program's native format) to ensure that the alpha channels are available if you need to edit the picture further.

Now let's go on. How do we turn this flat selection into 3D-looking slime?

First of all, let's fill the selection with a pure white color. If you prefer a more exotic color to your slime, just choose a color of your liking. I'm going to stick with white because it's more visible in this tutorial. Besides, it does look like cum.

Add a new folder named "Slime 1", then add a new layer in that folder named "Slime". Select the flood fill bucket and fill the selection. I've turned of the marching white and black lines so I can see the selection better. Use CTRL+H to toggle selections and paths (not used in this tutorial) visiblity.

But it's very plain, isn't it? Fluids and slime don't have the same opacity all over. They are more and less opaque depending on the thickness and the amount of fluid. They tend to be less opaque where there's less fluid and more opaque where the fluid is collecting, like in driplets and around the edges.

We'll fix that by using the eraser tool. Select the eraser and use airbrush mode and set the strength real low, around 5% is usually good for me.
We're not going to erase the slime, but we're going to make it more transparent where the fluid isn't collecting.

Carefully make the top parts of the slime more transparent with the eraser. If you erase too much, undo or change to the airbrush tool and - using the same strength setting, 5% - add some more white color to make the area less transparent.

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