How can we make a field mandatory in a form (created via RTC)

Use Preconditions and Followup Actions. There are two possible preconditions you would want to investigate

 - Required Attributes for Type and State

 - Required Attributes for Condition

Both of these will let you select fields to be mandatory based on the workflow or on a calculated condition.

It's important to note that 'mandatory' or 'required' in the case of EWM means a field must be changed from its default value, so you'll need to examine your fields and make sure they have an initial value that must be changed - a good example of where this is a bit of a problem is a Boolean attribute, where the default is false (unchecked) and so making it a mandatory field means it's got to be set to true in order to progress, which may not be what you want.