• write out melody with basic rhythm
• look for natural opportunities to swing and syncopate based on the lyrics
• rhythms that feel conversational are OK even if not connected to the actual lyrics
• force the issue by moving notes half a beat earlier or later
• common rhythmic idioms - Charleston, Johnny's Theme, etc.
• no need to commit to choices yet
• for songs in 3/4 or 6/8, decision time: convert to 4/4 or not?
• melodic variation welcome, in moderation
• more extensive melodic changes should wait until harmony is established

To create a score set up like this one. you can download the template here. Save it in your MuseScore3/Templates folder, then you can select it when creating new scores (it will be under "Custom Templates").

Or, use the "Jazz Lead Sheet" template (under "Jazz"), then go to Eidt / Instruments, change the Ordering to "Jazz Combo", select the existing staff, and click "Add Staff".

Other resources:

• Syncopating The Eight Basic Rhythms