。
She's wearing a leather belt at her waist.
Literal
She [topic-は] waist [at-に] leather 's belt [object-を] is-doing.
ベルトをする pairs the loanword ベルト with the all-purpose する — Japanese uses する as the default verb for putting on accessories: ネクタイをする ('wear a tie'), 指輪をする ('wear a ring'), 時計をする ('wear a watch'). 着る is reserved for clothing, while 履く covers footwear and pants; する fills in for everything else where a body-specific verb doesn't apply. The locative に on 腰 marks where the belt sits. Genitive の on 革 ('leather') turns it into an attributive — 'a leather (kind of) belt.'