Are you student? Is this your homework? If all answers are "YES", please read the book

what your instructor has recommended.

I'm not your instructor, but I can recommend to read the book "Time-Harmonic Electromagnetic Fields" from R. F. Harrington. It is very educational and easy to understand, yet mathematical exact.

You have to solve the Helmlioltz's wave equation which  in general case looks like

nabla2E + k2 E = 0

E - elerctric field distribution

k - propagation constant (in step-like waveguide it is the wave number of a mode)

This formula of the electric field is straight forward since the power flow density is p= EH where E is the electric field and H is the magnetic field. E and H are related by H= E/z0 where z0 is the eave impedance. In the air it is equal 377 ohm. If this power is confined in a wave guide with a cross sectional area A, then the total power flow will be equal to the power density x A.

This means that the total power w of the electromagnetic beam propagating in a cross section A= p A= (E^2/ z0) A,

Here A the area of circular tube =pi R0^2, R0 is the radius of the wave guide.

In case of parallel plate wave guide the cross section will be rectangular and equals the width of the plate times the height between the plates.

This is an estimation of electric field in the wave guide given the power transmitted in the wave guide. It is based on the poynting vector.

Best wishes