Adding Your Outlook Account Up In The Outlook App
First you will need to open the outlook app you have downloaded to your desktop.
The icon for Microsoft Outlook look like this

Step 1: Disable "New Outlook"
Whilst in the Microsoft Outlook app, navigate to Microsoft Outlook in the top left corner and untick "New Outlook"
Step 2: Account Set Up
Navigate to Microsoft Outlook in the top left corner, go to settings and then select accounts.
In the account screen select the "+" icon to "add an account"
Step 3: Email Settings
A window will open, if this screen says "not google?" in the top right hand corner click this, if not follow the on screen prompts to set up Microsoft 365 as below.
Select Microsoft 365, you will be asked to enter your email address and Microsoft password.
Step 4: Enable New Look Outlook
Navigate back to Microsoft Outlook in the top left corner and tick "New Outlook" again