This is going to be one of the bigger chapters, covering grammar, spelling, and paragraphing!

Proper grammar can sometimes be difficult to obtain, but with some practice it can get easier! For example, using different punctuation can make a big difference! It stops sentences where needed, and insinuates pauses where they're needed. Run on sentences should be avoided, as it only confuses things and makes it difficult to pick out one portion of the reply from another.
For example:

Johnny went to the market to get fresh vegetables and he saw where Joshua was working at the register and went over to Joshua saying hi where are the fresh vegetables can you help me find them

With some punctuation:

Johnny went to the market to get fresh vegetables. He saw where Joshua was working at the register, and went over to say hi. "Hi! Where are the fresh vegetables? Can you help me find them?"

The addition of basic puncturing makes the sentence easier to read, and easier to respond to! Adding in quotation marks ("") allows your rp partner to know that your character is talking.

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Next up is spelling! If you're not a native English speaker, that's perfectly okay! Some words are especially difficult to spell. My go-to for words I'm not sure how to spell is Google, or your preferred search engine. Simply typing the word into the search bar should result in Google giving you the correct spelling, or showing you that you got it right. Good spelling skills also help to make the response readable.

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Lines! Who doesn't love lines and responses? It goes without saying that you shouldn't reply to your rp partner with one sentence replies. It shows a lack of interest in the rp, and may upset your partner. Try to at least make three sentence replies, and work your way up from there.

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Next up is paragraphing! This is also important, as it breaks the wall of text, letting it be split into two events. If your roleplay only consists of three-five lines, you shouldn't worry too much about paragraphing. However, paragraphing should be used when switching from one character for the other, or making a new scene.
For Example:

Johnny went to the market to get fresh vegetables. He saw where Joshua was working at the register, and went over to say hi. "Hi! Where are the fresh vegetables? Can you help me find them?" "Sure they're over by the fresh fruit, I'll lead you to them!" Joshua responded, leaving his register. Once over by the fresh fruit and vegetables, Johnny picked out four bell peppers and three tomatoes. "Thank you so much Joshua!"

Corrected:

Johnny went to the market to get fresh vegetables. He saw where Joshua was working at the register, and went over to say hi. "Hi! Where are the fresh vegetables? Can you help me find them?"

"Sure they're over by the fresh fruit, I'll lead you to them!" Joshua responded, leaving his register.

Once over by the fresh fruit and vegetables, Johnny picked out four bell peppers and three tomatoes. "Thank you so much Joshua!"