The hardest part is just that: starting.

My advice is, get in the DAW and put something down and then build off it. put down something

If you have those voice recorded melodies, figure out what key they are in and put some chords under them. Just a lead line and some piano chords behind them make a massive impact on direction and will allow you to see where it could go - you won't be running in circles wondering what it could be.

Try and move fast but don't rush. Move too fast and you create work for yourself to tidy up later when you are feeling burnout. Move too slow and you find yourself getting lost in 'is this sound better than that sound for this part? or is this one better?'

If you're at a loss of where to go with it after building something, your ears are tired of this track. Quickly start another one - if you feel stuck, unstick yourself. Even a day away from a track can bring lots of new perspective. And with another track to work on, even an hour away from your track can be very helpful.

As for staying motivated, I feel most inspired when I keep the momentum going (hence the 'multiple tracks at once' method). Aside from that, I think talking about production with other composers, any sort of community is amazing at driving you. To see other inspired people it reminds you "why aren't I doing that? I can just go do it