Upwork is definitely easier to get clients. Now I receive most clients on LinkedIn, some of which even contact me out of the blue. But for the bulk, you’ll have to reach out and make yourself seen.
Cold calls and warm calls are one option. Build connections with people who could need your services in future. A lot of them don’t even know about Upwork. It’s especially useful to target SMBs this way and just let them know you exist.
It’s a bit of a slog to spend hours researching companies, but it can be worth it. Just don’t try to be selling yourself. Connect on a human level. Ask them if they ever used freelancers during their work. Tell them a bit about what you do. Some will reach out to you when they got something.
As for invoicing, it’s not as complicated as it may appear at first. You can find many templates for invoices online. It’s just a piece of paper with some necessary informations.
Check out https://invoice-generator.com
It’s a free tool to prepare a simple invoice online like a fill-out-form. It’s easy to use and does its job.
Just send the pdf per email to your client and make sure the email header mentions the invoice and what it’s for.
That’s about it.
But upwork is not bad for a start. Apart from the negatives I mentioned, I never made any experiences that were awful.
Let me know if I can help with anything else.