Firstly, you want to go to the SLim beta page -- Windows and Mac only. Linux users, you are out in the cold darkness. And then the ice-weasels come. Sorry. Make sure you're logged into the Second Life website, or later steps aren't going to work.
From there you will need a special First Look build of the Second Life viewer. You'll need to log into this at least once, and turn voice chat on while standing on some parcel with voice enabled. This apparently passes some sort of authentication token to Vivox that allows them to suck data (like your Friends List) out of the Second Life grid as they need it. If your viewer cannot connect to the Vivox voice servers, this isn't going to work.
Then you'll need the SLim client itself. Windows or Mac only. Linux users have all been devoured by the ice-weasels by this stage anyway.
Then you click on the button to "Create SLim account". You will need to enter an email address and create a password for using SLim. Don't use the same one as your regular Second Life password. Enter it, then a second time to verify, and finally press the Register button. Then run SLim and enter your avatar name, and the new password, and it should authenticate and connect.
And this is where things started to go sour.
The interface is, much as you might expect, plain and unadorned -- but the quantity of system resources that it uses are likewise trivial. A mere 40MB of memory for the Windows version, approximately.
Now... think of the very first Instant Messaging client you used. You know, back before they had any features. That's SLim. Voice and simple text messaging... and that's it.
Actually, technically it is even less than that.
My Friends List took about 10-15 minutes to load. Every now and again, a new name would pop onto the list. Each name is prefaced with an icon. The hand-and-eye associated with Second Life, or the SLim icon for SLim, showing which class of viewer the user is connected through. If the icon is greyed out, the user is offline.
This long loading (and names showing as base64 strings or as strings of digits) appears to repeat every time you log into SLim. Not heartening. Being logged in with the First Look viewer makes this go faster. No, really. Think about that. Obviously someone wasn't thinking things through.
With 25 friends online, SLim shows me... three of them. Also, it won't let me talk to any of them, as they are not using SLim or the Second Life First Look viewer -- apparently the person you are talking to must be using one of those two clients in order for you to communicate with them through SLim. Period. Also, you can more or less forget trying to contact anyone who isn't already on your friends list. If you want them to be, the only way to add someone is via the Second Life viewer.
Voice is, apparently, clearer through SLim than through the Second Life viewer -- probably because of the lower resource consumption.
SLim does allow logging and audio alerts, but the basic functional utility of being able to communicate with others is noticeably absent thus far.
As yet, there's no sign as to when (or if) Linux support will be forthcoming, or if source-code will ever become available for SLim.
Right now, we're underwhelmed by SLim -- almost shockingly so. Still, this is an early beta (we hope -- the version number is 2.1.3010.1755), and we presume that it will get better. Actually, it almost has to. It doesn't seem to have anywhere else to go.
UPDATE: Linden Lab has removed SLim and the setup process so that it is no longer available.