There are a few different ways that you can proofread documents online.
One way is to use a free online tool like Grammarly. Grammarly will scan your document for grammar mistakes and suggest corrections.
Another way is to use a paid online proofreading service like Scribendi. With Scribendi, you can submit your document for proofreading and receive corrections back within 24 hours.
If you want to proofread your document yourself, there are a few things you can do. First, make sure to read your document out loud. This will help you catch any errors that you might not catch when reading silently. Second, use a spellchecker. Finally, ask someone else to read your document for you.