Really broad question!

Most times I start out with an idea (eg: sound design/percussion rhythm) or more often, a melody, and try and put it as best I can into the piano roll.

From here i usually try and add chords, harmonies and possibly counter melodies. Usually play around with the key as well, and transpose it up or down. 99% of the melody and the actual musicality happens in this stage; all on piano (this does have its limitations. think about how many songs are amazing but are not the most complex).

From here I tend to flesh it out with a build up, break, a B section and an out, or a selection of these at least. Then i usually split the parts i've already come up with across different instruments, and perhaps change their parts slightly to fit their characteristics (eg: longer notes for pads, shorter ones for stabs etc (simplified)).

mixing happens right from the moment i have any more than one instrument playing with another, whether it be testing out melodies or trying to squash the percussion together. it usually happens from start to finish.

PRO If the original idea is what you wanted it to be; it's very easy to build around it and make a nice musical flow. CON: this may not sound like a con, but like i said, finishing the melody before messing around with the actual sounds/tonality of your synths sometimes limits how far you can go with this.

Notice how often I said usually. This isn't set in stone for me personally, and i'm assuming it tends to be different for almost everyone.

There's no right or wrong way, at all.

Hope this helps :)