"Say to the believing men that they should lower their gaze and guard their modesty: that will make for greater purity for them: And Allah is well acquainted with all that they do."
Educate yourself on what modesty is according to our faith and purify your intentions so you are pleasing Allah.
Buy loose fitting \ Underlayers
Underlayers allow you to put together a modest outfit with flexibility in the clothing you choose - also choosing loose fitting clothes keeps you comfortable and won't expose your figure.
Start looking at your closet through an islamic lens. Take out anything immodest and donate those clothes. Start buying pieces by prioritizing modesty over flattering fits and trends.
Shop Modest Fashion Brands
Shop from brands that fit the criteria of modest clothing according to our faith. When shopping for clothing it should be dignified and modest and not flashy that may defeat the purpose of dressing modest.
Behaviors and Manners
More importantly, one must be modest in behavior, manners, speech, and appearance in public. The dress is only one aspect and merely reflects what is present on the inside of a person's heart.