Adding a slideshow in a child-theme

[ Moderator note: moved to Fixing WordPress. ]

Hi! I would like to add a slideshow plugin to the theme that Im using. After doing some research I’ve tried multiple different ways to add the slideshow, all of course in my child-theme so that the added code doesn’t disappear when the theme is updated. The solution I’ve used (for now) is simply copying header.php to my child-theme and adding the code for the slideshow. I’ve tried using hooks in functions.php but can’t get the slideshow to appear where I want it (which is below the logo + global navigation but above the rest of the content). Which way is the best and most effective way to add something to a theme? Thanks in advance!