After horseback riding today, while grooming the horse, I carelessly got my right foot stepped on.
Literal
Today horseback-riding after, horse [possessive-の] grooming doing time [at-に] carelessly right foot [object-を] stepped-on [passive-れた].
乗馬 ('horseback riding') is the activity; 馬の手入れ ('horse grooming') is the aftercare. うっかり ('carelessly, absent-mindedly') is a common mimetic adverb. 踏まれた is passive of 踏む ('to step on') — the speaker is the victim of the stepping. The sentence is a diary-style narration of a minor mishap, natural and relatable.