How do I get a guitar shaped cake to stand on top of a round cake? Please help!...making this for daughters b-day on sat.

I would make a "real" guitar stand. You would need to use two cake boards - one flat on the round cake, the second under the whole guitar shaped cake. Support it with wooden or plastic dowels through the round cake for more stability.

Something like the attached picture.

You would need probably only the black supports, not the red one. I would give it there anyway.

Behind the guitar shaped cake on bottom, I would add small wedge from cake or foam, glue it with a bit of buttercream to hold it all together well.
You could even set the supports under the guitar shaped cake - real guitar stands look often this way.
You can disguise the supports with fondant / modelling paste.

Good luck to you!