This pretty easy task, if you can get a demo wireless access point and measure the radio signal strength based on the interior architecture of the building.
Download this android application "wifi Analyzer", this will help you get an idea about how the radio signals are spread across the area respective to the position you are fixing the AP, as well the interferences.
Mark the positions on the floor plan which you already must have.
If your wireless network demands a lot of data usage, say for eg: Voice/Video usage on high demand you have to make sure the particular area should have at-least a minimum signal strength with-in -55db. or else its fine you can space/position the AP's accordingly with a coverage of around -65db. I have practiced the same and has found the procedure effective
Or else if you only have the floor plan and No demo Wifi, You can generate a heat-map with the floor plan by using Cisco RFtool under this link
You will have the option to select the option to select the Wifi models which you may require and upload the floor plan select the building area, draw the floor plans and design a Wifi Heat-map by your own with in minutes and directly even export a BOm through CCW.
Hope this is helpful, Kindly let me know in case of any further clarifications @Israr