Following along in MyCase is recommended as you read the article.
Overview
The Single Select Custom Field lets you create a list of pre-defined items. When creating or editing a case, contact, or company record in MyCase, you can choose an item.
How to Create Single Select Fields
Name the Custom Field
Choose Single Select as the field type
Click Add list option.
Type a List Option into the box
Press the check mark
Repeat until you've added all the options you'd like
You can re-order the list options by clicking and dragging them up and down
Once everything is added and ordered correctly, click Add Custom Field.