People may need to know your OS first, I dont know if it is different from OS to OS.
In Windows you just click on the clone icon

, Then making sure you have the tool options open, assuming you want to clone from an image make sure your source is on image and alignment set to other than none or registered.
If from the same image, I use image as source and for alignment either fixed or aligned if you want to clone the same areas. You may have to test the other settings to see what they do.
There is also pattern as source, which would be your open pattern if using it.
With the clone and heal brushes they work the same as the paint brush. for a straight line you can click on shift for a straight line amd take your cursor to where you want the line to go to.
I have left multiply mode on before and nothing seems to happen. Leave it on normal unless you find a better mode.
Attachment:
Clone.png
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