1: Create a level overview screenshot, this video explains how to make one, the program isn't picky with file formats for screenshots: png, jpg or even tga are supported
2: Take note of the coordinates of the camera at the moment you took the screenshot (x, y and the cl_leveloverview zoom level), there are two different ways: cl_showpos 1 or the values displayed in the console when you use cl_leveloverview, note that these coordinates are different but the program can understand either of them.
Tip: You can use setpos <x> <y> <z> to position yourself accurately
3: Drag and drop the screenshot over the program's window
4: Drag and drop the demo(s) you want to use for the heatmap
5: Fill the camera coordinates and zoom level, don't forget to tick the checkbox corresponding to what type of coordinates you used (cl_showpos or the console)
6: The "Export image" button lets you export the heatmap as an image file

How to build

(This step is only needed if you want to build from source, if you're on Windows you can simply download a pre-built exe from the releases page)
Download and install Rust then cargo build or cargo build --release