Everything is explained very well here
https://housecurve.com/usage
Basically five steps:
1. download the HouseCurve app
2. download the sine sweep file to the A6, in order to play the test tones
3. use the HC app to generate response plots of your room (by «listening» to the sweep tones)
4. following this, use the app to create filters, e.g. to Harman or B&K targets. You set the frequency range and number of corrections (max 10) as well as max dB and Q for each correction
5. on the A6, enter these filter values as a correction in the PEQ-section
Advice on full range vs. low-end PEQ only varies. Read around on Schroder frequency for an idea of issues. Lower limit should be roughly the lowest extension of your speakers. From my very limited experience, and in my room, PEQ up to around 450hz and no more than 4db corrections seem to work the best. The good thing is that once measured (save the measurement in the app!), you can play around with different assumptions and generate a truckload of filters to test. Also, don’t forget to remeasure after applying filters to see effect. Also remember that cutting peaks is significantly easier than filling troughs, which in some cases may be nigh impossible.
Bear in mind that there is a lot more to serious-level PEQ, and that the real experts here will probably laugh at the banality of the above. Nevertheless, one has to start somewhere. I have found a huge improvement in sound (ie. the music I play is closer to what I would like it to sound like) by tinkering around in this fairly simple manner.