Tattoo cover up means covering up the old tattoo with new design, and the design of the tattoo is made from the modification of old tattoo.
I have friends who have gone through the process of cover up and have two different opposite results.
Let me start with good one, Pawan made his tattoo in his early age one in hand and another in his shoulder but he always used to wear long cloth to cover his hand and shoulder. The time when he had tattooed he was drunk and the tattoo artist was his armature friend who was drunk too and they make a tattoo by ink and cloth swing needle and the outcome was too horrible. These days he is happy since he covered his tattoo with new design.
( Pawan's Tattoo cover up Technique Before and After )
Another friend of mine has a tattoo of his girl friend name in his shoulder, which was big enough to cover the shoulder. And the time came, Suresh wants his tattoo removed since they were no longer in relationship, He went through tattoo cover up technique but it went wrong now he has black square box in his shoulder. These happen due to bad modification and artist.
Tattoo cover up technique depends up on the tattoo, small is good and the space around it could be modified or not.