Make sure your gains and x-overs are adjusted for the best most realistaic sound quality. Make sure you tone controls such as bass, treble, loudness etc... are all turned off. You want to make sure you not corrupting the freq responce in any way. If your using rear speakers, turn those off for tuning. Once you have your EQ set up you can go back and tweak later.
Get some graph paper, write down all the frequencies in a row with a space by them so you can fill in the SPL number. Then position the SPL meter on the center console facing forward or maybe a bit higher like on the top of the passenger seat. Set it to slow and C weighting. Adjust your volume on the test CD so that you can hear the low frequencies without overpowering your mids or highs. You will need to make a note at what volume you are using on your headunit so that these measurements are repetable. You might also pick a middle frequency like 2k and note the volume. That way if you adjust your gains and the volume number on the head unit changes you can play the 2k note and reset the volume.
Sit in the drivers seat and go through the tracks with the CD palyer or PC set to repeat the track. Calmly watch the SPL reading as it may not stabalize. I had to sometimes write down the average reading, but Iwas using a test CD, not the downloaded one mentioned above. Go from one freq. to the next and write down the SPL reading. You'll need to adjust the range on the meter as the volume gets louder or quieter.
Once you get all the freqs. wrote down you can use the graph paper and treat each line as one dB. Graphing them out will help create a better visual representation. At this point you may want to use the correction chart listed above. I never knew one existed, but I've only used RTA for the last several years. Assuming the correction list is correct, go ahead and use it.
Now comes the hard part. Looking at the curve and deciding what needs to be corrected. You can start by looking for peaks. If a single peak in the response curve (not the EQ curve) is higher than 3 or 4dB compared to the adjacent freqs, then drop it down to so it's within no more than 2 dB higher.
Then look for dips in the same way.
Your not trying to get the response curve (not the EQ curve) perfectly smooth nor perfectly flat. You just want to get rid of the peaks and dips. You want to use the least amount of correction possible which is why I didn't say to flatten the peaks or dips.
At this point you want to start making corrections in groups. No more individual freqs. At this point it gets real hard for me to describe the type of curve you want. You can try specific freqs you think need more volume by playing some high quality recordings. Stuff that was recorded accurately. Add a little peak at a specifc freq you think needs to louder. If it sounds good, adjust 2 or 3 bands on either side of that center freq. so that you don't create a big peak. You want the response curve (not the EQ curve) to be fairly smooth between the bands. I don't think it matters too much what the EQ curve looks like, but if you see a big dip or peak on your EQ curve, like 4 or 5 dB more than the bands next to it, then you might want to back that down or up a little. The reason being speakers don't usually create such peaks or dips in such a narrow frequency, so it may be the result of reflections or absorbtions in the car. You also don't want to over correct.
Hopefully using this technique will give you a nice, but not quite polished tune. It takes more time compared to using an RTA, but it's cheaper and you can do it yourself. Remember to keep the windows up and the don't let outside noises interfere with your readings. Make sure you save the EQ settings when your done. Just in case, you might want to go through all your EQ settings freq by freq and write them down. That way you can go back and adjust your EQ very quickly if you need to. Or if you switch EQ's the new one can be set up very quickly.
If any more experienced tuners want to add something or point out some flaws, please go ahead.