Okay so before I begin, Ima answer a couple questions that people asked in the comments.
1-how do you make stories like this? Well I usually have dreams of my stories and then write them out the next day.
2-How do you upload stories? First, if you're on a tablet, iPhone, etc then you need to switch to the desktop mode but if you're already on a computer then that's fine. So, to switch onto desktop mode you scroll all the way down to the bottom of the screen and in the bottom left corner should be some writing in kind of a darkish blue. Click on the word desktop and it'll take you to desktop. Once you're on desktop then you need to click on your username in the top right hand corner next to the Log Out sign. It should take you to an options thingy. Click on the publish option and go to Doc Maneger. You write your story under either DocX or Copy-n-Past. I always write mine under Copy-n-Past. Then once you're done you submit your document, do to publish story, read over the guidelines, then fill out a short form saying what story, crossover or normal, etc. Now, without further a-do, chapter 11!
They all looked around to check out their surroundings. "Hm, this isn't what Mewnie looked like last time I saw it." Marco said. "Yeah, Mewni has more castles and unicorns, this place is full of blackness and sadness." Star said. They were surrounded by hills that were covered in black mush, trees that were a dark grey color, and the skies were light grey with white clouds.
Mabel looked almost as if she were gonna cry. "This place has NO COLOR!" She ranted. Dipper told her to calm down and she did, after a while.
"If we're gonna find Bill, then we're gonna have to do it together." Stanly said. "But that portal took us to a colorless dimension, not Mewni." Star said. "The portal is built to take you to whatever dimension Bill is in." Stanly explained to the group. "Dude, Dipper, your uncle is like, really good at this kind of junk." Wendy said. Dipper laughed nervously and could feel his cheeks getting hot. "Heh, yeah. My uncle known all of the uh, ins and outs of things." Dipper bragged with his voice cracking and rubbing the back of his neck sheepishly.
Soos looked around putting his hands in the shape of binoculars. Mabel thought it was funny and did it with him. "Yo dudes! I see him."
Bill was in a colorless village and the villagers were screaming their heads off. Star looked up and saw her castle in the sky, it was in complete black and white colors and almost completely blended in to the sky. She gasped, "WE ARE IN MEWNI!" She yelled. "What!?" Marco yelled. "Yeah, that's Lenny! A guy who came to my 7th birthday!" Star yelled, pointing at a tall pimply teenager. "Bill must have either done this or Ludo got your wand and he's doing this!" Marco said breathless.
Sorry it's so short, I'm kinda in a writers block right now. Haven't seen any new episodes of either shows and there's some unanswered questions.