How do I do a 3D lion made of Fondant? And what to I use to make the design hard to stay in place?

Dont use fondant. Use gumpaste. If you have to use fondant, Add tylose to it which will make it set hard.

I did a tiger not too long ago with a fondant and gumpaste (50/50) mix. For a lion I would color the mix first. I made my tiger by shaping the individual pieces and then putting them together with gumglue and spagetti noodles. Make sure you use tissue to support the figure while it dries (which will take a few days). I set my tiger on some soft foam and used straight pins to keep the supporting tissue in place.
After I presented the cake to my friends I took the tiger off so I could keep him as a memento. He is rock solid sitting in my cake cabinet now.

Here's my finished tiger:
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1637178&sub=1637182