What exactly are the steps required for a hacker to hack and subscribe a drone that is currently subscribed to another hacker? Will this cancel the other hackers subscription, or can a drone be subscribed to more than one hacker? What if the other hacker is currently jumped into the drone that you are trying to hack?

Here is what I have been doing.

Electronic Warfare + Scan (Find the drones node)
Hacking + Exploit (Hack into the drone, which the faq says is sufficient for subscribing, admin access required)
If enemy hacker is jumped in, then cybercombat
Data Search + Browse (Look for file containing list of subscribers)
Hacking + Edit (Remove enemy hacker from list of subscribers)

Is this about right? How does spoof come in? Is it used if you want to send a command to a drone without actually hacking it?

this is one of the areas that regrettably the book dont go into much detail on.

yes, spoof is for giving single commands to a drone without hacking it fully.

something interesting would be to send it into diagnostics or maybe have it shut down.

if your gm (or you are the gm) accepts the test you describe above as acceptable for hacking a drone, go for it. right now the book is as clear as sewage on the topic...

Yeah, that sounds about right. Your choices are to attack the drone's node directly, or Spoof a command from the rigger. Of course, in order to spoof said command, you need to have the rigger's Matrix ID, and if you don't know where said rigger is, you'll have to find the drone's node (which is probably in Hidden mode), decrypt the incoming signal (assuming it's encrypted, and it should be), track it back to the rigger (who might be using Spoof, too), then analyze the rigger's node (which may be running Stealth) to get the ID.

Of course, it might be faster to just smack the drone with some Stick-n-Shock ...