It's incredibly intuitive to do (sarcasm intended). First click on the symbol gallery in the drawing. Then the easiest way is to click redefine in the menu manager and pick the first symbol you want to save. It'll bring you to the edit area, click done, then click write and it'll ask you what directory to save in it (it'll have a default already if you have that set up in your config.pro) and then click ok. Repeat for any other symbols you would like to save.