Let's use Rochelle for this. The tools used are shown in the pictures. When Ro is in Gimp, make a new layer.
Now make a rough shape of the cum. The actual shape will come later.
Fill the shape with white.
Now choose Gaussian Blur. The standard is 5 x 5. But I used 12 horizontal and vertical here. Use lesser blur on smaller cum shapes. The blur is not for the edges, but gives the white a certain depth.
Now remove the selector tool as shown below.
And here is the part were we are making the actual cum. Using the opacity and soft brush
size options, you can shape and form the cum, also making it more transparent in certain places. Also remember to adjust the opacity on the right after need. This is practice. The concept is easy to grasp.
And here is the end result. Now you see why the edges aren't important in the early stage.
Practice this and you will soon get the hang of it. The cum shape was bigger than it needed to be to give you more options as you remove, soften and reshape it.