FreeCAD is one of the free cad programs. FreeCAD releases new assembly add on. Firstly if you don't have it, you should install. Every version includes new future.
If you want to use assembly first you should install assesmbly add on, so you need to check it from toolbar. If you have it or install,you can start assembly.
Secondly, you should have some saved parts or you cad draw them in part design. There are many assembly drawing internet environment. Open assembly add on an click import button. Freecad asks you give a name for your file. Call any name then select all parts step by step.
If you want to understand clearly, you should watch this video below.
