Most expenses arrive on their own from your connected bank and card accounts. You enter one by hand when it did not come through a connected account — cash, a personal card used for the business, or a bill you are recording before it is paid.
Adding one
Open Accounting → Expenses and add it with the vendor, amount, date and category.
The two decisions that matter
Which category. This comes from your chart of accounts and it decides where the cost appears in every report. Be consistent rather than precise — the same vendor filed the same way every month is more useful than a perfect taxonomy applied unevenly.
Which job, if any. A cost attached to a job becomes part of that job's margin. The same cost attached only to the month tells you what you spent and not what it bought — see job costs.
Materials and subcontractors usually belong to a job. Rent and insurance never do — see cost of goods sold.
Exceptions
Personal spending on a business card. It belongs out of business expense categories entirely rather than filed into the nearest plausible one, because it distorts every expense ratio it touches and is the hardest thing to unpick later.
Ongoing rather than one-off
New transactions arrive continuously. Clearing the review queue in Transactions is the ongoing version of this, and it is where most categorisation actually happens.