Why rain works
Rain has most of its energy in the middle and low frequencies, like pink noise, so it covers household sound, traffic, and a television in the next room. The variation in real rain matters: a synthetic loop is easy for the brain to learn and then to notice; a long field recording never quite repeats.
Setting it up
Rain is the best choice for laptop speakers because it has less deep bass than brown noise. Set it just above the room. If you are in a loud café, you may need brown noise instead; rain will not cover speech at a comfortable volume.
Pair it with a block
Sound stops the room from interrupting you. The timer stops you from interrupting yourself. Twenty-five minutes with a written task and a chime at the end is the whole method; the rain is the part that makes it pleasant.