Step 1) Go to this site (CLICK HERE) and copy the code. Note: I do not own this endless scroll code at all, however I can not remember the original source, please let me know if you know the source.

Step 2) On your tumblr, go to customize > theme > use custom HTML.

Step 3) Now this is the hardest bit. You need to scroll through the code until you find the </head> tag. You need to paste the code you copied just above this tag… like this…

Step 4) Now you need to scroll down through your code just a little bit more to be able to delete the next/previous page buttons.. The code for the buttons will look something like this (highlighted)…

Step 5) Delete this bit of coding and save your theme & voila it should work!