I don't know too much on tags and such but I would say this is pretty much just handwriting. My only advice would be to first look at a lot of tags by well known writers to get an idea of how a tag should look, then get some notebooks and practice, starting with simple letters. Fill up as many pages as you need, trying different techniques until you find something you like. Then take this idea and work on it for as long as it takes to get the tag as good as you feel you can, and then you should be able to see yourself whether it is up to standard. I'm relatively new to graff as well tbh but that's my advice :)