Go to your Donorbox dashboard. By each campaign, there is a +Add Donation link (see image below). If you don't have a campaign, please create one. A popup will open in which you can enter your donation details such as your donor contact information, donation amount, payment type, and the answers to custom questions.
Please note: offline donations will not count towards your billing total. Your monthly Donorbox platform fee will not be affected at all.