It's perfectly simple really, and there are various methods which you can try. First, get yourself a normal Slitheen (it may take some persuasion) and after that, forcefully push it into a large bowl of baked beans to achieve the desired orange-glow. Another option is to cover the Slitheen in oil and let them sunbathe for a few hours, and they'll be crispy orange in no time. Ta da, you now have a Blathereen.
It seems the SJA crew have once again run out of ideas and created this alien. I'm going to be psychic, and predict that the following series will somehow involve the Trickster, or maybe a Graske, or another returning alien which has been done to death on the show.
Yes, how did you guess that I despise SJA? And yes, this is going to be used in the upcoming Series.