How do you use custom scenery in rct3?

i have downloaded many custom scenery like tunnels, hangars, stairs beside roller coasters, lights, rockin roller coaster items etc but i don't know how to activate or use them in the game. i want to use the stairs for example, but i don't know where to find them. i know there in my rct3 portfolio and i made sure i saved them, but what is the next step after you downloaded custom scenery onto your computer? {need of desperate help}