How to create huffman dictionary, encoding and decoding without using built-in functions?

I have a digital communication project assigned, i need help with one of the stages. the first stage is i need to format and image and then source encoded and decoded using huffman. the problem is that i can't use the built in functions, and i am to build a tree then a dictionary then encode and decode the image. i first converted it into a gray scale pixel matrix thrn i calculated the probabiltiy. I need help with huffman source coding, i need to build a code but i don't know how.

img = imread('sample.jpg');
imgOriginal = rgb2gray(img);

[rows, cols] = size(imgOriginal);
Image = imgOriginal(:);
 [N, M] = size(Image);
 Count = zeros(256,1);
for i = 1:N
 for j = 1:M
 Count(Image(i,j)+1) = Count(Image(i,j)+1)+1;
 end
 end
 p = Count/(M*N);