1. Click reply
2. Click Add Meme
3. Click Add Image (this is towards the top right corner of the meme template
that is currently displayed)
4. Choose whether your putting the image Inside the Current Image or Below
5. Choose where you're getting your image from - Upload Image is from your
device - Search Memes is from the IMGFLIP database. If you're using
Search Memes, you'll get more images returned on the search if you check
the box that says Include User Templates. When you do that you'll get a pop
up on your screen that warns about NSFW, click OK on there too.
6. If you chose to put the image Inside Current Image it will be on there, but a
bit smaller so you need to size it to the same size as the original.
7. Caption it
8. Click Generate
9. Click Post Comment OR continue to type text comments where it says type a comment.
NOTE: I've only been able to do this on my computer. If I try to do it on my phone, I have trouble getting the image resized. Others have said that they have no problem doing it, so I don't know, it could be me or my phone, whatever.
Also, you want the orientation of the original template that your importing onto to match the image that you're bringing in. For example, if you're using "One does not" as your original, that's Landscape, and if the image you're bringing in is portrait you're going to have trouble. So, switch your original to something like Face You Make Robert Downey Jr.
Hope that helps, let me know if you have any other questions.