If you are using the Midjourney Web UI, some of the parameters are accessible directly on the interface. However, you can also use parameters directly in your prompt. Here's a cheat sheet to help you understand the parameters you can use with Midjourney.
--ar width:height
Sets image width-to-height ratio. Default is 1:1
--no item1, item2
Tells Midjourney what not to include in your image. Add multiple items separated by commas. Using --no is more effective than saying "without" or "don't" in the prompt. For example, "still life --no fruit" works better than "still life without fruit".
Note: --no item1, item2 is equivalent to using a negative weight in multi-prompting (item1::-.5, item2::-.5)
--quality .25, .5, or 1 or --q .25, .5, or 1
Determines rendering time and quality. Higher values consume more resources. Default is 1.
--seed 0-4294967295
Sets starting point for generation. Same seed and prompt produces similar images.
--version 1-6 or --v 1-6
Selects the Midjourney algorithm version.
--c 0-100 or --chaos 0-100
Controls result variability. Higher values yield more unexpected outcomes.
--style raw
Applies alternative aesthetics. Raw offers less automatic beautification.
--stylize 0-1000 or --s 0-1000
Controls strength of Midjourney's default aesthetic style.
--fast or --relax or --turbo
Controls processing speed and GPU usage for image generation.
--sref
Uses a number or an image URL as a style reference to maintain consistent visual style across different prompts. Add "random" to use a random style from Midjourney's internal style library.
--sw 0-100
Sets the strength of style influence when using --sref. Higher values create images more similar to the reference style.
--cref
Uses an image URL as a character reference to maintain consistent character appearance across different prompts and scenes.
--cw 0-100
Sets the strength of characterization when using --cref. Lower values focus on face only, higher values include hair and clothing. Default is 100.
--iw 0-3
Adjusts influence of image prompts vs text prompts. Default is 1.
--niji 4 or 5
Generates images in an anime style.
--p
Makes the job private, not visible in public feeds.
--repeat 1-40 or --r 1-40
Runs multiple jobs from a single prompt.
--stop 10-100
Ends generation process early, resulting in less detailed images.
--tile
Generates images suitable for seamless patterns.
--video
Saves a progress video of the initial image grid generation.
--weird 0-3000 or --w 0-3000
Introduces quirky and offbeat qualities to images.
For a comprehensive list and detailed explanations, refer to the official Midjourney documentation.