Don't do this yourself, work with your attorney. If you don't have one yet, go get one who's experienced with startups.
Financing documents are not the kind of documents that you want to make any mistakes with.
Your attorney will be able to provide you with templates. You'll likely have a little bit of back and forth with your investors on terms, but the more standard terms/docs you start with, the less back and forth. If you're looking to minimize cost and you're familiar with writing/modifying legal docs, make some edits to the docs your attorney has provided, and have your attorney review them and make any necessary edits. Or just communicate the changes that you need to them and have them handle it.
After you've got your docs in order, there are other things you'll need to handle like SEC filings and sometimes filings with the state. Your attorney will know what needs to be done.
Cost will vary depending on what kind of financing you're doing (eg convertible note, ie debt, vs equity), but it's not too bad and well worth it. For a pretty standard convertible note, you're probably looking at a few thousand dollars. Making a mistake could cost you a lot more than that later on.