How to do I create a CustomSelectList Field?

Step-by-step guide

Select the Module link in the top left banner then select the appropriate module name link.
Select the Forms tab.
Then select the Master name link.
Chose a CustomSelectList that has not been used already.
Select the Edit pencil icon on the field.
Change the System Name and the Column Header to the name you want to assign to the new field.
Select the Options tab.
Click in the blue area to begin entering options.
Enter the option in both the Value field and the Text field.

To add additional options click on the + in the upper right corner.
Once you have entered all desired options click the Update button and then click Save on the Master Form.