I clicked the link and watched the tutorial you saw. Its the sort of tutorial that someone who hasn’t used blender or other compositing software would find hard to follow. Forgive me if what I’m about to explain is obvious, but I don’t know how well you know compositing software, so ill have to cover these concepts.
A mask is a black, white, and grey image that you can use to define which parts of an image to hide. In blender, you connect the mask image to the fac slot of the node you want to use it with. Ill use the mix node for my example. The black parts of the mask reveal pixels in the first slot, the white parts reveal the image in the second slot, and any shades of grey in the mask will mix the colors from the two images. Heres the type of workflow youll use when compositing.
First ill make a mask and add a node for it in the compositor (Hook it to a viewer node in order to check the result):
Next ill add a node to the line between the mask and the viewer in order to modify the mask, in this example i just blurred the mask a little, but you can add any combination of nodes:
Now that I’m happy with how my mask looks, I’m going to hook the mask to the factor of a mix node, and hook the two images I’m compositing together to the image slots:
With the mix node, we aren’t limited to just mixing two images together, it also has compositing options which change the result. I’m going to switch it to overlay mode:
When you are happy with the result, remember to hook the last node in the chain to a composite or file output node, the viewer node is just for checking on the changes. I hope this helped, and I’m sorry if i just said a lot of obvious stuff.