To register your expenses, you first need to create Contacts-Suppliers that can be named after the expense, e.g. "Transportation". These 'suppliers' will be available while you are add an bill or direct payment.
You can then proceed as follows
#1 By entering a "Bill"
You can enter your expenses explicitly (as a product/service) or by putting a generic title (eg office expenses) and proceed with the payment, just like you would do with any other purchase document.
#2 By adding a payment
Alternatively, you can simply create direct payments through the "(...) More -> Payments sent" menu, selecting "Add". The difference between this and the first option is that these records are not involved in the contacts' statements or the purchases reports. So you cannot keep track of the balances or include them in aggregated reports for expenses.
We recommend you to choose the first process, as it offers more options.