My Japanese class didn't teach how to make the sound; we were expected to hear it and eventually mimic it. Very few were able to actually do that.
I'm a fan of using linguistics to explain things, and I also like using analogies to what we have in our native languages (English for me).
For linguistics-minded people,
Japanese phonology - Wikipedia, the free encyclopedia They are all basically alveolar flaps, which means you flap your tongue against your alveolar ridge, which is the roof of your mouth where it is bumpy about 1cm behind the teeth, right before your mouth opens up into a wider cavern.
In English, the only alveolar flap I can think of offhand that we have is the "tt" in words like "better" and "butter."
らりるれろ is a little different, but it's close enough. It's definitely the closest you'll get when explaining purely with characters on a screen.
Personally, I've just always thought of it as a blend of "d" and "l."