Okay, so you've grown beyond having a stack of desktops, and instead of using 1U servers as impromptu breakfast tables, it's time to dip your hands into a dedicated room in your house, install a rack, pop the servers onto rails and get things done.
Note: This guide is a derivative of a forum posting I wrote a while ago explaining how to rack things.
First - take a deep breath and don't panic. Racking is not that hard.
What is a "U" in the context of setting up server racks? A "U" is defined with a height of 1.75 inches, or 45mm. The rack width is standardized at 19 inches, or 482.6 mm. Those 2 dimensions will never change. The depth (the distance between the front and the rear of the server) of the rack is variable, since you could have deeper or shallower racks depending on the rack manufacturer, and that's why the racking kits on the server will telescope (gets longer or shorter) to compensate for different racks in the market.
If you look at each side of the server rack you should typically see square or circular holes running down each pillar - they are arranged typically in groups of 3, like s0. Every 3 holes make up a U. Your rack are fitted with square holes.
Good server racks (like APC Netshelter) typically have markings denoting the U position, like so - as long as you pay attention, you will do just fine. Sometimes servers come with a cardboard rack ruler to help measure things out.
Next, look at the ends of your rail. How does the rail secure onto the provided holes, and how do you mount the server onto the rack once you are done?
So that depends on the rail family you are dealing with, and whether you got one of those fancier Enterprise servers, or a cheap no-frills server.
For example, here is the ends of a typical Dell ReadyRail set:
Notice that the rail ends have hardware pre-placed, so all you do is open up the rail so it'll fit the depth of the server rack, figure out which U you are dealing with, and just place it on in both the front and the back - there is usually a interlocking mechanism on them to keep one side of the rack from falling out once you place it on, and all rails have screws or latches to keep the rail from telescoping back open once you have it in place. With these setups you typically do not need additional reinforcements. The weight of the server and the interlocking will keep the server rack from falling out.
In some cheap server racks (Whitebox vendors like Infotrend or SuperMicro are guilty of that) you'll need to directly screw the rack into place. In this case, you'll need a partner to hold the rails on the front while you work on the back, or vice versa. If none are available, that's why Velcro tie-wraps exist to make temporary slings. Oh yeah, how do you screw something round into a square hole? Cage nuts - they go on the inner edge of the rack. Here's what they look like:
Rack nuts are held in place by the top and bottom flanges and are to be mounted on the side facing away from where the rack will go. You count off the top and bottom of each U, and pop them on/off as you see fit - it depends on the screw positioning on the rack - it's often 4 on each side. They are the bane of my existence as an admin as they are not entirely standard and are a pain to install/remove. Due to the typical layout of a data center they'll often fall into places where you cannot reach them (like an underfloor conduit). One of the in-jokes for a tech is the big bin of rack screws and rack nuts found in gigantic bins at high end data centers (like Equinix) near the break room, usually next to the ridiculous optics/cabling vending machine. They are usually discards from techs cleaning out cages, and it's a hodgepodge that might/might not help you when/if you drop one and it goes missing.
By the way, regarding those vending machines - they might be lifesavers (good luck finding a Juniper 10G SFP on a Saturday afternoon in the South Side of Chicago if the one you have on-site is dying), but they are almost always one-armed bandits (it's selling for $649 when you know they are $325 from other vendors or $200 when slow boated from China).
So here's a real life example of a typical rack meant for Proliant G5. There's a pair of inner rails, a pair of outer rails, and usually a set of screws if it is delivered with original packaging (make sure you don't throw it away. I had to scream at plenty of "Smart hands" coloc engineers for losing mounting screws in my past). You'll need a long Phillips head screwdriver, a calm and patient mind, a spare pair of hands (belonging to a helpful partner) or some velcro tie-downs if your spare hands are not available. The orientation looks correct. The rail on "top" is the right side (when you are on the front of the rack facing backwards). the one next to it are the left side rack (the orientation is typically engraved onto the metal).
First things first. The inner rails (if required) need to be mounted on the server via flathead Phillips screws. It should be short travel (around 4mm) and in the bag. Look at this diagram for a generic idea of what needs to be done:
Then you'll need to mount the outer rails to the rack and secure it. Something like this:
Every manufacturer is slightly different when it comes to how their outer rails interface with the rack. Some are just clips. Some are actually just a bunch of holes on a metal flange, requiring you to put in rack nuts. Judging by the photo, looks like the Proliant G5 rack already have the hardware pre-placed, so no need for rack nuts and all that unpleasantries. The 4 studs on each outer rail (2 in the front, 2 in the back) should go into the square holes and stay in place. Then you just lock the middle and call it a day.
Here's a youtube video of someone mounting a Proliant G6 2U server. The G5 rack is a little different in that the bar runs across the bottom U rather than the middle, so vertical orientation matters here. But it's generally the same principle - rack one end, go to the back, rack the other end, lock the middle, add cable management arm if present, then rack the server.
Grab a buddy and have him/her hold one end while you work on the other (Or if not, feed a velcro tie-wrap to one of the U holes and make a temporary mount sling). Just remember that the rails go on the inner edge and presents a continuous surface for the inner rail (mounted to the server) to slide against. Make sure that the Us are even and all on the same level. If your rack is unmarked, get a silver marker and delineate each U triplet - you'll thank yourself later.
Make sure that the racks are even and on the same Us throughout.
Trust me, make sure. Misalignment can damage rails, and that can lead to equipment drops. Dropping a Cisco Catalyst 4507 was one of the accidentally not-funny things that you'll not want on your job history, as me and an ex-coworker from a previous gig can attest.
Once you are sure, put some boxes under the rack (just in case you are not sure and don't want to take the gamble), and then slide the server in.
Here's a generic look of what it should look like:
Oh yeah, I should mention that not every server rail set will have removable or discrete inner rails. The Dell ReadyRails have their inner rails integrated into the rail set - they can slide out but not all the way. To mount them, telescope the outer rails open to mount the rails to the rack (correct U and all that), then latch it to prevent it from telescoping back open. Then pull the inner rails out all the way until it reaches the limit. on the top of the rail you should see 4 captive notches, which corresponds to 4 pegs on the chassis itself. Hold the server up until all 4 pegs align with the notches, place it down and into the notch, and then once it engages the inner rail, push the entire thing back in. The R620 that I admin at work are mounted like that - good stuff.
Alright. Hope that was helpful. Good luck.