If you are building workload estimates or planning by month (e.g. I'm assigning 50 hours for Sabrina in project A), here is the suggested approach:
1. Add an assignment to a placeholder
From the Project Planner, identify the project
Click Add person or placeholder> add a placeholder or person
In the placeholder, create one assignment spanning from the project start date to the end date, across multiple months
you can drag and drop or use the calendar tool to select specific dates
use the default effort hours, we will adjust that later
2. Split the assignment
Switch to the Quarter view, so you can easily see the gey lines to identify the last day of the month)
(Click the image to enlarge)
Right-click the assignment > select Split
Split on the last day of each month. Repeat until you have split all the months.
Use the arrow keys on your keyboard to quickly navigate across the timeline
3. Add the monthly scheduled hours for each month
After splitting all the months, click on a monthly assignment
From the Quick Editor, enter the total effort the placeholder will work for that month—for example, 50 hours—then click 'Save'.
To see the total effort displayed in the Planner by default, click the cogwheel icon on the planner and toggle on 'Show Total Effort':
4. Clone the assignments to another placeholder or a person
Now that the monthly assignments are set up, you can clone this structure to other placeholders or people and adjust the monthly Total Effort individually.
If you are cloning the assignments to another placeholder
Click on the 3-dot icon next to the placeholder name > select Clone
Adjust the Total Effort field for each month for the new placeholder if needed
If the new placeholder should be in a different project role, click on the 3-dot icon next to the new placeholder name > Select Change Project Role
If you are cloning the assignments to a person
Add the people to the project
In the placeholder with monthly assignments, find the earliest assignment > Right click > Select All to Right
Right click again > Clone All Selected
Click Clone next to each person's name from the list:
Adjust the Total Effort field for each person as needed