How do I display data contained in a separate form based on user input?

How do I display data contained in a separate form based on user input?

I have two forms. One (called "Writers") contains a list of freelance writer names and their "Rate Per Word" (a currency field). The other (called "Clients") contains a list of clients, and includes three fields that are relevant to my question:
  1. "Writer": The writer assigned to the client. This is a Lookup field that is linked to the Writers form.
  2. "Total Words": How many words are written per month is entered in this field.
  3. "Monthly Cost": The monthly cost of the writer assigned to the client, calculated by the writer's Rate Per Word multiplied by Total Words.
The Monthly Cost field needs to be calculated by the relevant Rate Per Word field listed within the Writers form multiplied by the relevant Total Words field contained within the Clients form. How do I do this?

I hope that makes sense; thanks in advance for your help!