Solving PDE with Euler implicit method

I want to solve the Swift Hohenberg equation for homogenous solution, using the Euler method.

This is the function I have for the Euler method:

function [U] = Euler(f,y,dt,tmax)
% func - is a function handle
% dt - the steps
% tmax - the maximal time
% y - the first guess

% parameters:
q = 1;
b = 1.8;
r = 1;
f= @(u) r*u - u.^3 + b*u.^2 - q.^4*u;  % Swift Hohenberg equation

y = -1:0.1:1;                          %initial guess
dt = 0.1;
tmax = 10;

t = 0:dt:tmax; 
n = length(t);
U = zeros(length(y),n); 
U(:,1) = y;

U_old = y(:);

for i=1:length(t)-1
    U_new = U_old+dt.*f(t(i),U_old);
    U(:,i+1)=U_new;
    U_old=U_new;
end
end

I'm not sure about the initial guess I have.

I really don't know what to do from here. Any help would be appriciated.