There is a lot to remember and when just starting out with the system, expecting any single person to be in full possession of all of it from the start is unreasonable. That said, adopting only parts of the rules at one time, or just narrating the abilities and results of the professions and aspects you are not prepared to run yet will lead to inconsistencies in the campaign and open you up to the risk of accidentally misrepresenting the flow of the game to the players.
- My solution to this is to divide the labour.
Generate, or have volunteers generate cheat sheets for the specific systems of the player characters. Review these with the appropriate characters when you can. The goal here is to familiarize yourself with the procedures and expectations, while training the player at the same time. Between the two of you, things should go smoothly in play. As sessions progress, your knowledge of the system will grow and your reliance on the player's assistance will lessen. As an added benefit, the player will more quickly become adept at using the character's skills and abilities and can then focus on personality and immersion.
- The second step is to explicitly practice.
In the early stages balance your sessions to allow for scenes that focus on 1) using specific aspects of the system (start small and get progressively larger) and 2) specific aspects of the setting. Expose yourself and the players to practice with the mechanics, and to interaction with the world and its characters.
- The third step is to know what is possible, and to know what is possible now
Unless you have a huge group, you will not have a full representation of character professions and types at your table. As everyone develops a stronger awareness of the setting, their thinking will likely expand incrementally to include options which they cannot perform, or have not yet seen. If you really are not ready to include an aspect of play in a session or scenario, be prepared to explain why it was not possible in that particular case. Don't rule it out, or just invent a result, build up the background of your tale by knowing what got in the way of that option. If you start out without using magic, eventually the players may wonder why they never meet any mages. The explanation helps make the setting yours, and it builds anticipation for the day the mages do show up.
- The last tip I have is to accept that this game will take time to learn
Build good locations, interesting missions, and compelling NPCs. Encourage well-developed personalities in the PCs with room to grow into the setting and establish roots that may not be obvious at the beginning. Take everything in stages, but start from a solid overview of what the 6th World holds.
The system is daunting at first. It gets better.