The only thing required when making a sale is that you have created some products/items in the inventory. You cannot make sales without products/items.
To make a sale you must:
Go to Checkout
Tap on an item that you have listed. You can also search or scan the product/item with a barcode scanner
If you want to change the product/item text, price or discount on the item, click on the item in the window to the right, which is showed above.
Press Pay
Select payment type
Print or email the receipt if you like
Manual sales/miscellaneous sales is made by having a product/item that has no saved price.
When this product/item is selected in the box, a new sektion will appear where you can specify price etc.
For this, it is a good idea to have created some items at no cost: e.g. "Workshop", "Equipment", "Clothing".
This way you do not destroy inventory and statistics on items that have been created correctly in advance.