i only just figured out how to do it
for anyone else who doesn't know, here's how to do it:
1. select your first meme
2. go to the buttons above the image, find the one that says 'add image'
3. click on it
4. in the pop-up window, select that you want the image to appear underneath the current one (it will automatically say inside the current image)
5. go to the bit underneath, and decide your second meme - you must know whether it is featured or not.
6a. if it is featured, then you can search for it by typing in the name of the template in the box, i.e. One Does Not Simply, etc.
6b. if it is not featured, then provide the link or click on upload to upload it.
7. having selected the second meme, you can now scroll down to hopefully find the second meme underneath and in line with the first.
8. you can repeat this process to make a great chain (like the awesome one above), or stick with just a couple.
9. as long as you have the images in mind, you can make any meme chain you want
10. enjoy!