Download the spreadsheet:
If you're not familiar with the Elitist Jerks forums, you should educate yourself sometime soon. They're the nexus of some of the most progressive and comprehensive class discussions and theorycrafting around. I wouldn't recommend jumping in with both feet forward, however. Create an account, lurk for a few weeks, and then enter the discussion once you've gotten a feel for how the boards work. They're very strict with their content and posting rules, so please make sure to read their guidelines to ensure that you're falling within their approved posting format.
Once there, I spend most of my time reading the Rogue-specific class discussion board. You can find a lot of great information here about specs, gear, rotations, etc. You'll also be able to find Aldriana's spreadsheet thread, with basic instructions and download links. This will be your first stop: go download the spreadsheet, open it up, and continue reading.
There's a lot of information here, but once you get the flow, it will be much simpler to manage. Basically, listed here is every piece of decent Rogue gear available at 80, Ulduar included! At the top of the sheet, we find helms, followed by amulets, etc. The trick is to work your way down this big list of gear, and mark the gear that you are currently wearing. This is also where you'll select your enchants and gems.
The only area where you should edit is the "Equipped" column. For each type of item slot (helm, shoulders, chest), put a '1' next to the item that you are using (or considering). If you are not using a particular item, simply clear the cell (leave it blank). Ensure that you are only marking 1 item per slot, so that the spreadsheet doesn't think you're using two pairs of boots. The score value for each item also gives you an idea of each piece of gear's relative strength. This tab is where you'll spend most of your time.
I am using the Combat spreadsheet as a reference, as the Mutilate sheet lacks this tab. In the Combat sheet, we see two options: Instant Poison vs Wound Poison, and Envenom vs Eviscerate. Selecting your choice is done in much the same manner as it is selecting gear: just place a 1 next to your choice and clear the other option. As a rule, Mutilate uses Instant MH and Deadly OH, while Combat uses Wound MH and Deadly OH. With the buff in 3.1 to Deadly Poison, it will be in the offhand for every Rogue spec. Combat can fluctuate between Wound and Instant, depending on if you spec into Improved Poisons or not. Most Combat Rogues will be using Wound Poison, so leave this at default.
Mutilate will again always be using Envenom, as the new Master Poisoner and Deadly Poison buffs make it too attractive to pass up for Eviscerate. Combat can choose between the two, depending on your spec. 18/51/2 is the Poison-heavy Envenom spec, while 15/51/5 is the more common Eviscerate spec. I find 15/51/5 to be better, based on both usefulness and rotation solidity. 18/51/2 leaves Relentless Strikes only partially filled, which can cost you a lot of energy in the long run. For soloing, 15/51/5 is also the stronger suitor. I suggest trying both and sticking with the higher performing spec that meshes well with your needs.
The Combat sheet again has a few extra options, namely the ability to choose which Glyphs you would like to implement. The Mutilate sheet assumes you're using Mutilate, Rupture, and Hunger for Blood: these are far and away the best 3 Glyphs for the spec. Combat has a few options, and they're mostly fight-specific. On fights where Killing Spree is inappropriate, you can use Adrenaline Rush or Slice and Dice, for example. Typically, you'll want Sinister Strike, Rupture, and Killing Spree.
As a Mutilate build, this tab gives you the option to choose between the standard 51/13/7 and a Lightning Reflexes build (Mutilate-Combat as it is also known). It also gives you the choice between Turn the Tables and Master Poisoner, as Master Poisoner is currently a viable choice even in the event that another class is bringing the 3% crit buff to the group. To choose your talents, simply put the number of talent points you're using into the first column. The second column is the maximum number of points for that talent, for your reference. Be sure not to put more talents into this tab than you have available.
Combat is again more flexible in its spec, and therefore you have a few more options for talent point choices. The one key to the Combat Talents tab is the SnD size that you'll see at the top of the chart. This value will let you know how many Combo Points you should use for your Slice and Dice ability. The chart assumes a xs/5r/5e spec, which means that you'll use 'x' combo points on Slice and Dice (s), 5 combo points on Rupture (r), and 5 combo points on Eviscerate (e). When you want to reference Envenom, the Rogue community has fluctuated between (n) and (v), but I see (n) being used more commonly now.
If your SnD Size on the spreadsheet is 3, this means you should always use 3 combo points on Slice and Dice. If it's 2.5, this means that you should use 3 Combo Points, but may be able to sneak a 2 Combo Point SnD in and still maintain your rotation occasionally. The SnD Glyph greatly affects this value, so consider that as well in your Glyphing choices. Maintaining your xs/5r/5e rotation is key to maximizing your raid DPS.
The calcs tab is the one area that you should definitely not touch unless you are 100% certain you know what you're doing. It does provide a very nice chart (at the very bottom) that will show you where your damage is coming from: It will break down White Damage versus Instant Poison Damage, for example. This can help you better understand how your spec is balanced versus another. Against a boss with high physical mitigation, a Poison-heavy spec would triumph, for example (though there are no fights quite like this).
So now you understand the basics of the spreadsheet, it's time to start plugging in new gear and enchant choices and seeing how it affects your DPS value. The key is experimentation, trying out different gear combos and seeing which returns the highest value. Remember that the sheet does make several assumptions about your gear, talent, glyph, and playstyle choices: it may not apply 100% to the way that you play your Rogue. However, it provides a solid tool to be better informed about how your character works and how gear and talent choices interact with your DPS. Next week I will go over specific instructions for choosing new gear and talents, using the skills that you've learned here today.