There probably isn't much else to say other than that said in the link Shuumai has offered. However I'd like to offer this personal perspective.
Breathing or knowing I can secure an air supply would be my greatest reason for being able to relax. Until breathing confidence becomes the norm, relaxing on command is difficult. And we all know how important relaxing is when related to swimming. The other day was only my second time at the pool since early December ... the skiing season owns "my" winter ... so the visit was spent trying to get back to whatever ability I had in December. Once I get a handle on confident breathing all else will be practice. So... last week I took a very slow stroke with concentration on getting a good roll for air. (Maintaining streamline position etc. is matter of fact or necessity...) And low & behold it started to work. Nothing pretty for sure but effective in that I managed a pool length on a couple of occasions. And though it was air to the left I was surprised since before this all my efforts were to breath to the right. I suppose this shows my roll isn't as symmetrical as it should be. I rolled far enough (not to sweet spot) to enable a relaxed inhale and exhaled completely (under water) before the next inhale. I had to stay focused, relaxed, slow, in line as much as possible and likely a bit over rolled. With practice the degree of roll will decrease. I had to push myself to complete a length. The key, is to push yourself. Expand the comfort zone.
I sometimes practice the stroke (relaxed hands, high elbow, entry timing etc.) while using a snorkel ( no nose clip or mask) and can travel back and forth all day. So my "plan" is not only to increase ability or technique is small bits but to also decrease use of the snorkel in small bits too. Eventually it will all come together. At least I hope so. I hope this helps and we hear from others with the "at home" drills your requested.