In this video, I cover the basic concept of flow validation, the difference between validation rules and flow validation, and how to use complex logic to validate one field based on multiple field logic.
Help doc from SFDC: https://help.salesforce.com/s/article...

Here is the formula used: this will vary depending on your condition and fields.
Use this only for reference.
IF(
OR(
{!Input_Text} = 'Standard',
{!Input_Text} = 'Enhanced'
)
&&
{!Picklist_Val} = 'Test1',

OR(
{!RX_Outside_Tier} = '$1,000',
{!RX_Outside_Tier} = '$1000',
{!RX_Outside_Tier} = '50%'
),

true


)