Specify increase amount, how it is applied (e.g., to base salary or bonus), and the effective date of the increase.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I'm pleased to inform you that, effective <effective date of increase>, your salary will be increased <amount of increase>. This will bring your salary to <$amount>.
We appreciate the fine work you're doing and are glad to have you as part of our company.
encl: <List of enclosed items goes here>