。
I make a point of writing to my mother once a month.
Literal
Mother [to-には] month [per-に] one time letter [obj-を] write [so as to-ようにしている].
~ようにしている is the 'make a habit of' / 'try to [consistently]' pattern — the volitional habitualization of a behavior. Contrast with ~ようになる ('come to X' spontaneously). The には on 母 marks mother as a contrastive topic — 'as for mother (specifically), I make sure to...' suggesting maybe other people don't get the same treatment.