This will vary from app to app. Look for instructions on the website you're using. Often times, they'll be a start button or a button with a pre-programmed song at the top or at the bottom of the page. Look around.
Using the default keybinds: Press "Shift +P" to open up your collections window. Then, click the "Mounts" tab to see all your mounts. You can drag one of those to...
These types of songs are called "remixes". You will have to recreate the song, but you can search up the melodies/harmonies/etc. of the song to copy them to your song.