Which one is correct?
You can start making the changes in your design.
or
You can start make changing in your design.
Which one is correct?
You can start making the changes in your design.
or
You can start make changing in your design.
Your first sentence (making the changes) is correct, because the changes are the things being made.
Because the changes are things, they must be nouns. It can then be determined that your second sentence (making changing) is not correct because "changing" is the gerund form of the verb.