I wrote recently about starting with the bass line when beginning work on an arrangement. That is generally a true statement. However, there is another technique which can be useful when mapping out your arrangement.
Work from the obvious.
That is to say, once you have all the correct notes roughly entered into your arrangement, go through and place a few (or many) phrases into the obvious voices. You're not stuck with those choices - changes can be made later. However, a bass clef melodic idea below the staff, or one that hovers near the top of the treble clef staff (for example) have only a few viable basic arranging solutions (excluding for a moment the ways you can color those solutions, i.e. mutes, octave transposition, doubling, etc.).
Adding the nuance (articulations, dynamics, phrasing) to those more obvious instrumentation choices is also a good idea. Sometimes this leads to suggesting how light the accompanying voices should be (or how heavy they can be).
When I get stuck, I'll start at the beginning of the chart, make a choice about how to start (where does the melody go), add then add the nuance to that melody. This often snowballs into other choices about the opening phrase, and I'll notice a few hours later that I've already worked through half the chart.