MSK Modulation & Demodulation Without Using MATLAB Built in functions ?

i need to do draw the constellation diagram of a bitstream modulated using msk and subjected to some noise

the trick is the professor imposed that we dont use the wgn, mskmod, mskdemod functions

i made some attempts to mimic the mskmod function but i didnt understand it very well to do so

also tried creating a for loop but it didnt seem to work at all

can anyone provide some pointers on how to msk modulate a small random bitstream generated using :

% Random bitstream generation
onezerowieghts = 1; %likelihood of 0's = 1's
bits = round(onezerowieghts*rand(1,5))