It is very difficult for us to give you a walk through as different landing pages have different code. However if you want to put the locker after a user has clicked a button, then you must track the button in the landing page code and replace it's link with your content locker link. I advise you to be careful with ripped landing pages, make sure you know what you are doing.
you need to look through the index.html/php file line by line
identify which line is responsible for the content locker
then cut it out and paste yours
shortcut to finding out which one it is..... open your page in browser .... right click -> "inspect element " on the locker...see what that line is called....find it in your text editor.
sometimes you won't be able to do it or it won't look right unless you are using the same content locking network as them....in that case easy trick is make another html file... put your locker on it....then iframe it in the landing page file.
if they have any analytics tracking codes in there, it's good idea to remove that too.