The Reddit conversations worth answering.

Draft

I make an invoicing tool for freelancers. For a one-off gig, the basics are your name and contact details, the client's details, a unique invoice number, the date, a line for what you did and what it cost, and the total. Payment terms and how you want to be paid help too. A simple template works fine, a lot of people start there. If you'd rather not build one, my tool creates and sends invoices to clients. Either way, send it as a PDF and keep a copy for your records.

Draft

I make an invoicing tool for freelancers. On your actual questions: decide what to charge before the work starts, not after, and put the number in writing. Send the invoice when the work is delivered, or on a schedule if it's ongoing. A deposit up front is normal for bigger projects and protects you. For the first few clients, a clear PDF with your name, their name, a line for each item, the total, and a due date is enough to look professional. Tools help with the sending and tracking part. Mine creates and sends invoices to clients. Plenty of others do the same.