Open the operation adding section.
Then enter the following:
- account that has been charged for this expense;
- the category that you want to relate your expense to;
- the amount of the expense;
- the date.
For best expenses detalization you can put tags on each operation and make comments.
Press Shift+3 to put a tag (you don't have to switch the keyboard layout, you'll get the sign "#" anyway).
After adding a tag you can make a comment on the expense.