Can someone give me a lead to writing invoice?
You mean writing an invoice for people who order your gig? You don’t have to, Fiverr sends a mail with an invoice to the customer, the customer pays Fiverr, not you directly, and once you delivered and the money cleared, you’ll get it. The platform is pretty low-effort paperwork-wise, thankfully.
If you’re talking about writing invoices generally, to clients you have outside of Fiverr, and don’t mind giving Fiverr more of your data, you could give And.Co a try, it’s a platform that belongs to Fiverr, and offers “Invoicing, Contracts, Proposals, Expense Tracking, Time Tracking and Task Management” and it’s free, at least for now. There’s a whole forum category for it: https://forum.fiverr.com/c/and-co