My question is, whats the best way to get it into the good range (~5.3?)
There are lots of good ways and which you choose depends on what you have available and your requirements. You need to do two things:
1)Overcome or remove the alkalinity of your brewing water
2)Overcome the alkalinity of your base malt(s).
I can get my hands on some conc. HCl, is it okay to just add that to bring the pH down,
Anyone can get 28 Be' HCl at the hardware store and use that to overcome the mash alkalinity. The question is as to whether or not you want hardware store acid in your beer. You can also buy sulfuric acid at the auto parts store but the same question arises. The use of these two acids can simultaneously combat alkalinity and adjust sulfate and chloride levels. They are widely used in Britain for these purposes but in Britain food grade acid blends are available. In the US not so.
HCl isn't particularly dangerous but you can hurt yourself with it for sure. Concentrated sulfuric acid is another story as it badly want's your 'precious bodily fluids' and will rip the water out of any part of you it touches. It is hard for home brewers in the US to obtain either of these in food grade and so I don't recommend that most people use them unless you have a food grade source and are trained in handling strong acids.
..or what are the most common methods that won't affect the flavor of the beer?
Probably the best and certainly the easiest (if you have a source) is to dilute the alkalinity away with RO water and then use a bit of sauermalz to overcome the base malt alkalinity. The reason this is the easiest is that the dose of sauermalz is so easy to calculate. One percent of the grist per 0.1 pH drop desired generally does the job. Sauermalz does add flavor but it is very subtle and is often sought by lager brewers. Concentrated lactic acid sold at LHBS is about 11.8 N for nominal mash pH. A typical base malt will need a little over 6 mEq/pound and so for 10 lbs you'd need about 60 so that approximately 5 mL are generally required. You need, therefore, some method of measuring small volumes fairly accurately and a syringe works well for this. Most LHBS sell 10% phosphoric acid which is about 1 N and so 12 times the volume relative to lactic acid is needed but it is easier to measure a large volume than a small one.
One can also dispose of much of water alkalinity by boiling it or treating it with lime. These processes are more difficult to understand and carry out than just dilution with RO and depend on the starting properties of the water but many brewers use them with success.
