Domain is a part of Elementary Types with global type, which use at all Data Element.
Steps to make Domain
1. Open t-code SE11
2. Activate Radio Button 
3. Insert Domain Name, initiate with letter Z or Y
4. Click Button 
5. Fill in Short Description. Then fill in Data Type, No. Characters, Decimal places and Output length like below.
6. Click button 
7. Package is filled to make Transportable Domain into other client, but you don't need to fill it if practice only.
8. Click button to make Transportable Domain into other client. Click button 
for practice only.
9. Click button 
to activate Domain