[Trackrash]

Draco makes a lot of good points.

First thing I would do is confirm your corner balance is good. There are threads on this. I use a modified "tripod" method.
Also, important, is the condition of the bushings and bearings. Loose or worn bushings or loose bearings will give unreliable readings.

You first need to verify your garage is level, if not get some rigid vinyl 12"X12' tiles from home depot. I used a HF 6' level and my digit level on top of it to verify zero. This part IMHO is the most critical to obtain proper readings. Mark the floor where tires need to rest, if floor is not level.

This is critical, and no concrete floor is perfect.

I used the string method and made a camber tool using modifying a digital level to fit my 17" rims. I am assuming you do not have HTH fancy laser tool so you now need to verify your rims are true or you will get false readings, spin them to check for hi & lo spots by using a hand drill to prop it up and place the tip against the rim ( use blue tape if you do not want to scratch the rims ) tape off hi and lo sections, you want to measure in the same spot of in the same sections always. Write down your readings.

Good point to verify rims are true. Also don't take measurements from the tires, only the rims. If you are running Fuchs, they are usually very true, but still you should check.
I use a machinist's level on the center of the wheel, where the cap goes, (obviously the center caps are removed), to measure the camber.

Raise the car and remove the wheel, spray the plates with some light colored paint at the side ways wide U and at the 2 adjustment nuts, allow it to dry then use a sharpie and mark around the U and pick 12 o'clock on the two nuts. This is now your 3 reference points of your current settings.

Good idea to mark where you start.
HOWEVER, I never raise my car before or during an alignment. I want the suspension to be "settled" in its as driven position.
I use a long 19mm combination wrench to loosen the nuts with the wheels on the ground.

To remove pre-load from the T-bars, move the car forward and back several times and bounce on the bumpers, front and back, wait a few minutes then take your readings.

Actually, if you don't jack the car up, you will still have to do this. This is a critical, and frustrating, step. The problem is your wheel's alignment will be held in place by the friction of the tire on the ground. Rolling the car allows the wheel to move into its new position.
Every time you make an adjustment, tighten the nuts, roll the car back and forth, then take your measurements.

A couple of other things to keep in mind. Changing the camber or the ride height can affect the toe setting, and vice-versa.

Remember that the rear wheel's toe settings must be equal on both sides of the car. In other words, the direction of the rear tires must be same when compared to the center-line of the car. Otherwise you car will not track straight down the road. Using the string method will help insure this.

Good luck.