Yea I change up my process everytime too. Usually start with sequenced drums, but I'll also start with a synth/melody line or some keys or maybe a breakbeat or other sample.
I do try to use pre-setup templates when i'm producing in Reason. Once I've got the first element laid out (whatever it is - drums, melody, bass - whatever) I'll just keep building on it.
Just keep in mind the basic parts of most music - rhythm section (drums, guitars, bass, etc.), lead melody section (synths, keys, guitars, etc), harmony/accompaniment sections (other instruments/sounds, pads, etc), some sprinkles of sounds to add diversity, etc.
Whatever you start with, just keep building out the other parts on top of it. Then when you've got a full loop that sounds good, start arranging it into different sections (intro, verse, chorus, bridge, etc.) You can color code your stuff however you want (color code by arrangement or by instrument sections) to stay organized when looking at everything. I need to get better at doing that to my projects.
At this point I'll listen to the track as a whole and go back over different sections, change out sounds you don't like, add other ones, introduce drops and other change-ups, sound effects + other ear candy.
After I've got the whole thing arranged and I like where it is as a full composition I'll get into the mixing and mastering of the track. Just my basic process