I want to make an ifr flight plan for A320 based on fcom, but I don't understand how to make one
The process of determining the optimal cruise flight level can be quite complicated. Multiple factors play into which FL is possible and practical. They include: Airspace restrictions, weather (wind, clouds, turbulence), terrain and aircraft limitations, among others.
Which altitude is optimal also depends on different factors and ultimately is the solution to an optimization problem for a mathematical function describing the relationship between aircraft range and velocity, altitude and aerodynamic factors. To finally arrive at the optimal cruising altitude, one needs to strike a balance between all these different factors.
If you want to get a flight plan with a good cruising altitude for flight simulator purposes, I would recommend using online resources like SimBrief.