Not hard at all, you just need a mic, a cable that connects it to a computer and done. You can use a RS spl meter without correction files to get you in the ballpark. I use to use my built in soundcard and spl meter for a long time and they were not that much different than my calibrated mic and sound card. Basically a RS spl meter plugged into the computer input jack and your output jack going to your processor. REW is free and people could help you with setup. If you have to buy a Mic still just do it right and buy a calibrated mic and sound card.
After I get the sound card and use my spl meter where do I go to find best placement for the sub?