At the time, I had to think of what kind of maps were achievable yet recognizable in the timeframe that I had since I was not familiar on how to use unreal engine. My mind went immediately to arena style maps, specifically those of the Halo franchise.
After considering maps like Guardian (Halo 3) or Warlock (Halo 2), I then concluded on my favorite map from Halo 4 being Haven. Haven is a simple symmetrical design that has great flow for combat and yet not too small for it to be considered claustrophobic or too chaotic.
The first step in my process when approaching this task after deciding on the arena was to gather reference images to get a better grasp on how that arena looks. Whilst gathering that material I realized I had direct information on the map since I owned Halo: MCC.
This was the key needed for me to not only get every angle I required of the map, but gave me relative sizing from the players FOV.

Front/Entrance View

Rear View
The second step was to begin constructing the layout of the arena from the default unreal player FOV.
I primarily used basic cube brushes for a majority of the maps design to maintain aspects of the original arena having a similar blocky/brutalist style.
The third step was to add finishing touches to the arena with custom brush geometry to nail down parts of the map.
As shown in the final product video, I was close to getting the map on a 1:1 scale, however certain areas were slightly bigger than the original.

Top Down View
Final Product: