It's basically random. I think of some combination of letters then I see what the rule has. If I found a property, I simply try to improve it. (Like when the rule has an almost spaceship, I find a simple transition change to make one with a spaceship) (If some still life is almost indestructible, I sometimes might want to not ruin that while editing the rulestring) But, I don't want explosive rules so if there's a replicator or quadratic growth, I try to stop them from working by either adding or removing letters from the rule. After all that, I basically find nothing that great. Most cool properties are found at random.
To answer your previous question more correctly...
After a while searching spaceships in rlifesrc, I see components that are easily connected... Wait why am I talking about this now? Ah yes, after reading this:
Now what does frothing puffer mean?
Probably a natural thing in my rule.