Enter it
Bills → New bill. Pick the vendor, put in their bill number, the bill date and the due date, then the lines: what it was for, which expense account, and a project if it belongs to one.
Saving it opens accounts payable — the business now owes this money and it shows in your cash flow.
Paying it
Record the payment against the bill. Or, when the payment appears on the bank feed, categorise that transaction with Find match and pick the bill — that settles it without creating a second record of the same money.
Seeing what is due
Reports → A/P aging groups everything you owe by how late it is. Cash flow shows the same money against what is coming in.
Bills against a project
Tag a bill line to a project and it lands in that project's actual cost, which is what makes budget-versus-actual real rather than a guess.