1. You hire an expert to do a wireless site survey; or 2. Do-it-yourself (DIY).
If you're not familiar with this activity it will take time to learn how to do this right. If you get someone to do it for you, you can tag along and learn from them. Being human, the expert tasked to do this like to brag and show you the functions and how to interpret the data so you learn. This is how I learn to do site survey myself even though my employer refuses to fund my training.