Help with first time kitchen

Hi guys currently going through my first housing project , I'm 26 years old I've been working as a maintenance technician for the past 7 years so I'm quite handy around the house , I'm halfway through fully redoing the house out, I've now come upto the kitchen and after doing my research I've still got a few unanswered questions so hopefully you guys can Help,

1. Do I need to take measurements while exsisting cabinets are still in place? Or could I literally gut the kitchen now before...

Help with first time kitchen