IMHO, the Spirit is the ultimate paper airplane design.
I consider it my Mona Lisa. To me it is a blend of art, science and spirituality.
If you can fold a Spirit paper airplane, you will feel like an artist, an engineer, and possibly even a paper airplane deity.
Spirit is a complex design that will challenge your skills. First bit of advice, you need to keep the tolerances loose. The folds must be even but not too close. Avoid the straightforward method of folding one side and then the other. It inherently changes the balance via the dynamics of compound paper folds. My advice is to fold one side, unfold it, then fold the other, unfold it, then fold them together at the same time. Last but not least, a perfectly straight tail is critical. I recommend folding the tail up prior to folding wings down.
In any case, take your time and do not give up. Learn it, love it.
Finally, I would like to acknowledge James Minoru Sakota for inspiring the Spirit design with his award winning 1966 origami paper airplane.
For the super geek like me, try my catapult modification (instructions below). You only need a rubber band and paper clip.