At a time
Definition 1 of 2
Pro Tip 1/3Sequential Tasks
Use 'at a time' to emphasize focusing on one thing before moving to another. 
I'm reading these books one chapter at a time.
Pro Tip 2/3Manageable Portions
It implies breaking down tasks into smaller, more manageable parts. 
We're renovating the house one room at a time.
Pro Tip 3/3Patience
Suggests patience or taking things slowly. 
You'll improve your English skills one step at a time.