I never do anything randomly! I'm one of the crazy obsessive people so I give a recipe a lot of thought before trying it.
With a SMaSH, I put together things that just sound good. Like Maris Otter malt and amarillo hops- I love them both so did them together. I also am wild about Vienna malt, and so I did a double decoction lager with Vienna malt and hallertauer hops (not so simple, really, but tasted great!).
One thing that really helps with making SMaSH beers and other simple recipes is that you can really see what a certain hop or certain malt brings to the recipe so in the future creating recipes is simpler.
If you like the description of a hop variety, try it in a SMaSH! Or if you opened a package of hops and loved the aroma- that will work great in a SMaSH!
I found out the hard way that I don't like citra as the hop in the flavor/aroma!
I'm an excellent cook, so I thought brewing would be easy. And it has come pretty easily in some ways. But it took a LONG time for me to be able to write recipes confidently, as it does take some experience to know what each grain tastes like and how much is too much in any given style. I guess that's also like cooking- I love oregano, but would use a teaspoon in spaghetti sauce, not 1/2 pound. The same experience will be needed in brewing- if you love aromatic malt, trust me- 4 ounces is better than a pound in most cases!