"The Filling Station" describes a small-town family-run gas station, and defines it by the dirtiness of the family and sons who run it contrasted with a doily and plant that seem unnecessary. The narrator, a first-person voice who for the purposes of this question can be deemed Elizabeth Bishop, is amazed at the care that goes into the small piece of decoration in a gas station that is both completely utilitarian and very dirty. The first three stanzas explicitly mention just how dirty everything is:

Oh, but it is dirty!
--this little filling station,
oil-soaked, oil-permeated
to a disturbing, over-all
black translucency.

For a diary entry from the narrator's perspective, you might focus on any number of small details; the Father's "monkey suit" that "cuts him under the arms," for instance. Is this a jumpsuit? Why does it cut him? "Monkey suit" is often used as a pejorative for fancy dress, as a suit-and-tie or tuxedo. Why is it used here? What does the narrator think about the Father using his sons for labor? Their ages are not mentioned; is this significant?
The "dirty dog" which sits on the back porch seems to have no purpose other than to continue the theme of dirtiness; why is it mentioned? Simply because a dog on a porch indicates domesticity?
The narrator, at the end, seems confused that a family gas station might include a mother. She interprets the doily and plant as larger signs of human love and devotion; "Somebody loves us all," she says, equating the unseen hand of the mother with the unseen hand of a deity. However, does she assume that the family lives in a constant, unending state of filth? Is that the life of the lower-classes who serve people such as herself? Or has she simply never considered the need in working people for decoration or escape from their daily lives?
Finally, you might take an entirely different approach than that of the poem. Examine the gas station not as a dirty, lower-class place, but as a necessary and efficient part of daily life. People don't often think about how hard mechanics work to support their families; is the Father being harsh to keep his children in the business, or is he trying to ensure that they have a solid foundation in hard work and skilled labor for their future? Perhaps in her diary, the narrator reexamines her preconceived notions about what hard work truly means, and what her first reaction says about her own prejudices.