Step 1 - Create a BEX Variable

Done directly in the Query Designer.

Step 2 - Create an implementation class

1 - Use Transaction SE24.

2 - Create a new class inheriting of ZCL_BIU001_VAR_ABSTRACT

TECHNO - BI TEAM > Variable Exits - Using ABAP Class instead of include ZXRSRU01 > image2016-4-5 14:7:7.png

Select the method according to your need.

Select the right method & click Redefine.

TECHNO - BI TEAM > Variable Exits - Using ABAP Class instead of include ZXRSRU01 > image2016-4-5 14:8:12.png

All former exit relevant parameters are available as attributes of the superclass

TECHNO - BI TEAM > Variable Exits - Using ABAP Class instead of include ZXRSRU01 > image2016-4-5 14:10:42.png

Go to transaction SM30, Table ZBIU001_V_VAR

TECHNO - BI TEAM > Variable Exits - Using ABAP Class instead of include ZXRSRU01 > image2016-4-5 14:11:49.png

TECHNO - BI TEAM > Variable Exits - Using ABAP Class instead of include ZXRSRU01 > image2016-4-5 14:11:58.png

Don't forget to transport the table entry (A customozing order is automatically requested) :

TECHNO - BI TEAM > Variable Exits - Using ABAP Class instead of include ZXRSRU01 > image2016-4-5 14:12:7.png

Processing of I_STEP = 3 is still in the exit. As it is not dependend on one variable & is used for specific purposes.
Don't redefine the method INITIALIZE : It's only used internally for parameters creation.