One thing that helps me greatly is thinking about the three T's Timbre, Tempo, Time
Timbre (the qualities of a sound)
. for an instance, the sound of a glockenspiel has a very brittle timbre, meaning that on its own, it has a nice beautiful and calm feel. Put it together with a bassoon or an organ playing in the lower register though, and the juxtaposition creates a creepy vibe.
Tempo (How fast you play)
something played fast is generally more stressing and busy than something played slowly.
Playing a song slower, and with fewer instruments, is a good way of making a night theme.
Time (What metre you are using)
4/4 and 3/4 feel normal to most of us. 3/4 is especially good for royal sounding music and water levels in my opionion
unusual meters like 7/8 or 11/8+13/8 is great for boss music
I'll give you an example. In one of my projects, I made a leitmotif for vibraphone, guitar, percussion, string section, brass section, choir, and electric bass.
For the main area, The song is in 4/4, and the guitar has the melody, and the background is held by percussion, choir, and bass. During the B part, the brass comes in, giving it some extra "umpf"
everything else is resting
In the forest area, the bass actually plays the melody, with the guitar strumming lightly, both showing what chord we're in, and giving some rhythmical information that we'd normally get from the drums. This leads to a very minimalist and empty sound, which works well with the forest vibe. To give it some more body, the vibraphone plays a counter melody in the upper register in the B section, a small part of the choir sings a single note, to make it less boring for the ears.
In the underwater area, the time changes to 3/4 and the vibraphone takes the melody, only making slight changes to work with the time change. The percussion is still there, but much more quiet and mellow, yet is only thing keeping us grounded and making the 3/4 time noticeable. The bass only plays on the first beat of each bar, and the strings swell in and out.
By giving each instrument a "mission"and thinking about what implications that timbre would bring to the song, you can change the song freely.
Did this help at all?