Apparently the code I posted doesn’t work anymore, if you just copy paste it, even though it is the same code in this tutorial. Weird.
So, I’ll post how I did it. With screen caps and all. YAY!
(Message me if it works or not or if you have questions!)
First step: Go to your ask page! (YOURURL.tumblr.com/ask)
Next: View the source code of your ask page. (For Chrome, View > Developer > View Source. For other browsers, IDK. It’s probably the same though.)
Next: Scroll down and find the code to your ask box! (Faster way? Ctrl+F and look for the title of your ask box. The code is right next to it.) Copy this code.
Next: Paste the code to your about me section or custom HTML. (For this, I pasted it in the about me section. I assume those that want to put it in their custom HTML know where you want it to go.) Change the width and height as you please. (For this, I only changed the width.)
Last: Save changes and see it for your self. Hurrah!