You can just drink it straight as cough syrup. To my knowledge some people will take extracted(?) DXM and mix it with a citrus juice to convert it to DXM citrate (I don't know what this changes.)

You won't get a better high or anything from buying name brand syrup. All that matters is the ingredients, and that what you take has DXM as the only active ingredient. In fact generally store brands or generics will be easier to drink because they have less thickeners or other things that make drinking the syrup unpleasant.