The requirements are:
Maxed Fortress
Maxed Heavy Assault
It is also recommended to have:
Maxed infantry production
Maxed vehicle production
Maxed Assaults
A small to medium sized map
Option 1: (Rank 13 and below)
Option 2: (Rank 14 and higher)
3 Barracks
Upgrade HQ to level 2
Optional: 1 Power Plant depending on energy production
Upgrade all Barracks to level 2
Optional: 2 additional Supply Centers
The main goal is to hit the enemy army as soon as possible, which is why a less economy heavy approach is recommended and why gathering sufficient containers is important. That's why the tactic should always be attempted with 3 Barracks instead of the more standard 2 Barracks approach as it will ensure that one can not only get more containers, but also makes getting map control for the early game easier to achieve, which can be very helpful as it will keep the enemy from scouting the rush, therefore making it a lot easier to pull off.
Both options are more or less identical in how they are played. The only difference is when and how many troops will be available for the rush. So choosing which one to go for depends mostly on upgrades like resource production (Supply Center), power production (Power Plant) and starting resources (Headquarters).
A good way of judging if one is able of going for option 2 is to play a training battle and pay attention if the resource production is enough to produce Heavy Assaults and Fortresses without idling any production building. Otherwise there is no point in adding them and simply going for option 1 is the better choice. Most players will start to satisfy this requirement around rank 14. Depending on their Skills and the boosts that are used this can vary though.
As soon as the build order is complete, it is important to start producing both Heavy Assaults and Fortresses as soon as possible and start gathering an army. At this point is important to keep scouting the enemy base regularly to find the best opportunity to attack the enemy base. If the enemy leaves their base completely defenseless (for example to go for a tech rush), it can already be enough to send in a small army as soon as possible to defeat them. If this is not the case and they for example have a Sniper Tower, it is recommended to wait a little longer and gather a bigger army first. It's all about the timing, which needs practice.
Usually there is no early game hard counter (which doesn't mean it is invulnerable) to this unit combination as both anti-infantry (Fortress) and anti-vehicle/building/Helicopter (Heavy Assault) are present, making it very well rounded against any kind of early game units. The greatest threat to it are usually bases with a lot of pre-built early game Sniper Towers and, if one is too slow to attack, Hawks, against which this army is more or less defenseless due to having no dedicated anti-air.
If such a thing happens it is recommended to not force the rush. Instead one can make sure to keep one's army at the edge of the enemies vision range, to make them think an attack is still incoming, while using the distraction to go for another more economy focused strategy like Zeus with Avia Support, which is where it comes in handy if one opted to build those 2 additional Supply Centers in the beginning.