Set templates for generating thanks for donations received. These can have your letter head and signature. They can contain pictures and text. Update this from time to time, to keep your thanks as appealing as possible.
Capture donations or import them. Import from bank statement, accounting package or spreadsheet. You would then go into each contact or organization and issue their thanks. A merge occurs for the first name, last donation amount and donation date. Edit the email first to personalize, or just send. Try and show the donor that you appreciate their donation.
You can send yourself a test thank you email before sending to the donor. This will help you get comfortable with the system. It is useful to do this if you have made changes to the template.
Once you are comfortable with the process, you can use automatic thanks.