Hello, userluke-januschka.
Adding a sale is very easy to do in QuickBooks. I'll definitely show you how to do it.
There are two ways to manually add a sale. You can either do it through an invoice or a sales receipt.
Simply click the + New button, then choose Invoice or Sales receipt.

Your choices depend how you are paid. If the customer paid on the spot, select the sales receipt. If they want to pay at a later time, use an invoice.
These articles can also contain more helpful details about a sales receipt or an invoice:
Would you like to customize your sales forms to make them more presentable and professional to your customers? We have a guide to help you out: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Let me know if you want to record other transactions in QuickBooks. I'd be more than happy to show you some steps or guides. Looking forward to your post soon!