You put env_cubemaps in each place you want the engine to sample reflections from (an example might be one in each of a number of small rooms), compile the map, run it in TF2 and type the following in the console:
mat_specular 0 - otherwise you get pink/black chequerboard textures on reflective items which will be compiled into your reflections
buildcubemaps - builds cubemaps
mat_specular 1 - once all the cubemaps have finished building
If you have HDR lighting you'll need to compile cubemaps with HDR on, then switch to LDR lighting, then compile again, then switch back to HDR and re-enable mat_specular. Once you've built the cubemaps, the reflections are embedded into the map file and will show up for anyone you give the file to (as long as you don't rename it). You will however need to rebuild them after a new map compile.