I suppose the very first thing I should cover here is the difference between writing the plot and writing the story. Despite having similar names those two words mean different things, the plot is the sequence of events in chronological order, while the story is quite literally how the story is told.
For example if you want to explain the plot of Final Fantasy VII, you should really go Crisis Core Final Fantasy VII-Final Fantasy VII-Advent Children-Dirge of Cerberus. Whereas as we know the way the story's told is VII-Advent Children-Dirge of Cerberus-Crisis Core Final Fantasy VII.
The point is, there's this thing I like to call "Writing the plot as opposed to writing the story" basically what it means is...Actually it's easier just to show you an example, so here is how writing the plot looks:
And then Makoto said "I love you" and kissed Shiro on the lips, before both fell to the ground and made out.
See the scene above has no depth to it, there's nothing to really pull you in and all in all it's just bland and boring, so now let's write a story:
Makoto's dark brown eyes shimmered in the pale moonlight as she looked at the man before her "Shiro..." she said calmly, and yet with an apprehensive tone "I...I love you" she finally admitted both to herself and to Shiro, before leaning forwards and slowly kissing him.
See what a difference that makes? It not only adds depth to the scene but it pulls you in, and makes it more interesting while not only telling you about the characters and where they are, but I digress.
The next point I want to cover is to do with paragraphing, I'm sure you've noticed I do it a lot...Some would even say too much, but it works, a massive wall of text is not only less likely to be read by people, it's also more likely to cause eye strain, I'm not asking for perfect grammar and sentence structure like starting a new paragraph everytime a new line of dialogue is made, but smaller chunks of words are easier for the reader to digest, which is a good thing.
I suppose I should talk about the Mask itself, one thing that should be noted is that there's at least three or four different interpretations of the Mask to choose from, such as the original Comic, the two movies the Cartoon and the comics influenced by the movie/cartoon.
One key thing that's present in most depictions is the idea that it warps the person who wears it psyche, for example the A-typical mask wearer is a shrinking violet who becomes a loud, crazy party girl, but as every person is unique, so too should be every mask wearer.
I sort of covered this last time, but I'll try and be clearer by using examples. Let's take two popular characters with similar dispositions, Hinata Hyuuga from Naruto and Orihime Inoue from Bleach. For starters Hinata has a number of family issues to play around with, picture a daddies girl from hell if you will, you don't have to make the crush on Naruto her key character trait.
Meanwhile for Orihime, you've got a plethora of weird ideas and gags to mess with, consider the time she said she wanted to be a killer death robot when she grew up, you could have her be the most wacky and crazy Mask wearer we've ever seen, with once again the love aspect being played down.
I can't really talk about jokes considering we all have different taste in jokes and gags and if you try to force a joke you aren't comfortable with, the awkwardness will show in your writing, so I guess my best advice is to just go with the flow when you're trying to be funny, think of what makes you laugh and try to work it in somehow so that it feels natural.
There's not much more I can say except have fun and do what you want to do, in my experience trying to write a story which you aren't fully invested in is pointless, whether it's Mask stories or original content you shouldn't try to force yourself into writing, let it come naturally and you'll find yourself in the flow of things, and able to write much more then you thought you could.