Type a (append) to insert text to the right of the cursor. Experiment by moving the cursor anywhere on a line and typing a, followed by the text you want to add. Press Esc when you're finished.
Type A to add text to the end of a line. To see how this command works, position the cursor anywhere on a text line and type A. The cursor moves to the end of the line, where you can type your additions. Press Esc when you are finished.