1. Use Grammarly. That'll help with periods, mispelling, and forgotten capitals. Most of the stuff that qualifies as grammar.
2. Tense. Check to make sure your verbs are the correct tense and form. Check your adjectives too.
3. Use the right words that make sense within the sentence. Do not make up nonsensical expressions. For example, 'she sat off the building' makes no sense.
4. Sentence structure. Make sure they aren't dangly long things with no proper form. They have to have a... hm... goal? A single thing you are trying to express. Do not frankenstein sentences together, or make them so short they mean nothing.
5. Sentence alignment prone to misunderstandings. For example, if Clara said 'Hello', you want her name or dialogue tag to be close to that piece of dialogue so it is clear who she is talking to. But if you write, "Hello." Raven screamed, "Help!" Then things become confusing even though you technically haven't done anything wrong.
Are you ok? That looks horrible.