How do you enter ASCII characters?

You type it. - in a way that is correct, but perhaps not over helpful.

ASCII = American Standard Code for Information Interchange. Over the life of computers, there have been several standards for encoding letters and numbers into binary strings ('0' and '1'). Early computers only had 4 bits for each character, this has slowly increased to the current 8 bits per character. Each standard has a name, ASCII is the name for the arrangement most commonly used today.

Though that is not quite accurate, first there are two ASCII codes, there is 7-bit ASCII and there is 8-bit ASCII, both with the same name. Also, most people use Microsoft Windows, and Microsoft do not actually follow the ASCII standard completely, some of the characters in Windows are different to ASCII on other platforms. Hence a common name used of 'MS-ASCII' when network technicians need to define which they are using, or converting.

If you have a modern small computer (Desktop, Notebook etc with Windows, Linux or MAC) it uses ASCII already, you do not need to do anything to get it. If you happen to own an IBM mainframe then this uses EBCDIC coding which is different - but the system includes all the code needed to automatically convert files between the various codes for you.