Most people on this forum seem to think that the in-game PS Editor is not working, but that's not true. At all. It got fixed for NS 3.0 Beta 3 (IIRC)
This isn't specifically to learn using the env_particles_custom entity. But if you want, there's a guide here.
So, how do you edit a PS ingame?
Well, you can use listps to find the name to your PS, but since you defined the name in the env_particles_custom, you should now that already...
Anyways, open your console and type "editps mycoolps" without quotes, and replacing mycoolps with the name of your env_particles_custom (obviously...)
Now, what we did back on the NSTR2 days was to type toggleeditps in console, right? Yes, but this isn't NSTR2 and we didn't have Steam (it looks like an issue of the new console)... so, in order to make it work bind toggleeditps to a key, I have it bound to F3, if you don't know how to do it, open your console and type: "bind F3 toggleeditps". You can change the key if you want, obviously.
Now, close your console, and hit the key you have bound the command, and enjoy the editing
Oh, and it'll take a while until NS "starts" the particle systems, so don't worry if you load the map and don't see them instantly.