Step 1:   Create a file called euler_simple.m whose contents are:

        function [x,y] = euler_simple(x0,y0,xfinal,n)

        h = (xfinal - x0)/n; 
        x(1) = x0; 
        y(1) = y0;

        for i=1:n, 
           y(i+1) = y(i) + h*f(x(i),y(i)); 
           x(i+1) = x(i) + h; 
        end

Step 2:  Create a file called f.m  whose contents are:

           function yp = f(x,y) 
                yp = x + y;

Step 3: Solve the problem using Matlab by typing

        x0 =0; y0=-0.5; xfinal=2; n=40; 
       [x,y] = euler_simple(x0,y0,xfinal,n); 
       plot(x,y)                                                       % We also see the solution plot!

A better way is to put every thing above in the file called main.m.