I think the people responding to this with some equivalent of "read more!" are missing the point a little. That would be closer to going to an art museum and looking at Old Masters paintings and trying to think of what you like about their styles—Which can be valuable, yes! But isn't grinding.
Grinding would be word sprints. NaNoWriMo was killer for this before their whole operation went to shit, but the good thing about that is that they never had copyright over the concept of writing fast. The numbers you go for can be a little arbitrary at first as you figure out what specifically works for you, but 15 minutes is usually the average from what I've seen? Maybe you can get 500 words in 15 minutes. Maybe you can only get 200. Either way, you're exercising the muscles that get ideas from your brain out onto paper.
Here's some other ideas I came up with for more specific exercises:
Wordsketching for Settings
Go outside (or stay inside?), practically anywhere. Sit down and angle yourself in a way that lets you get a good view of where you're at, and then pick the first things that jump out to you about the place you're in and describe them in 2-3 sentences. Then, figure out what you're drawn to next and write sentences about that. Make sure you add what you feel in that moment (Is the atmosphere uncomfortable? Is the place hot or cold? Humid?), what the place sounds like (People talking, or cars, or animal noises...), so on and so forth. This is very similar to the 5-4-3-2-1 grounding technique, so you can even work off of that structure if you'd like, but this is specifically to help you get better at describing settings.
As far as fanfic is concerned, I think the genre as a whole skimps on atmosphere and setting a SHIT TON. If you want to elevate yourself, let the pacing of your work slow down for a second and dedicate some time to really orienting your blorbos in a specific space. It does a lot for the tone of the work, and... if you want to play ball with the big boys you need to start thinking about things like tone and atmosphere when writing.
Transcription for Dialogue
This can entail you writing down what your family says at the dinner table like a creepy weirdo, or transcribing movie dialogue or anything of that nature. The purpose would be to get used to how conversation naturally flows between two or more people (turn-taking, interruption etc.), and furthermore getting a sense of how human beings naturally talk to each other. I've written down things I've heard people say in public before for no better reason than thinking "Oh! I could totally see [X Character] saying that!". Whatever hangups you may have over this sick linguivouyeristic perversion are much less important than the tragedy of filling your work with Incorrect Quotes -style unreadable garbage. It's like eating your vegetables.
Wiktionary Challenges for Word Choice
Just spamming the "random word" button on Wiktionary will give you instances in every language Wiktionary has in its database, so this one requires a little more work to be helpful, but in lieu of that you can look through stuff like Categories or Thesaurus Entries!
Let's gather a few fun words to demonstrate.
Okay, this should be more than enough for some sentences describing a setting out in nature! Blunket is making me think this takes place at twilight... And I like alliterating, so let's mash it to make the semi-redundant phrase "blunket-blue" just for some flair.
The copse laid still. On the bank of the meandering creek, the rabbit flattened herself out against the soft grass, her luculent eyes darting back and forth over her surroundings. What she could see of the copse looked mostly the same: Twisting shrubs the color of ash, blunket-blue grasses, an invisible wind brushing over all that stillness and pressing flat everything it could. Her nose twitched. Until the last blushes of rose drained from the evening sky, the rabbit would keep herself fixed in that spot on the riverbank and wait.
Boom! Those words are suddenly in your arsenal to use whenever you want, and you additionally have a reference point to remember what they mean if you forget. I think the only thing I could add from here is that reading your work aloud can help you monitor the way you chunk out your sentences and clauses, but that's more closely related to editing help and, again, not grinding exercises.
gummygunk
turnnoffyourmind
stormborns
blakekathryn
prguitarman
