Overlap of segments while manually implementing STFT

actually i have a large audio signal and i need to implement stft manually , i am succesful with my attempt but i have a problem with my overlap of segments

please kindly suggest me in solving this problem

thanks in advance

W_total1 =[];%nullmatrix

while (pos+nw <= nx) 
% while enough signal left

if n == 1
    w1start = 0;
    w1(end) = w1;

elseif n==2 
      w1start = (n-1)*w1 - feed;
      w1(end) = (w1 + feed);
else
    w1start = (w1start + w1(end))/2;
    w1(end) = w1start + w1;

end
z(n,:) = x1(pos:pos+nw-1).*w(:,28000);
  n = n+1;

    pos = pos + nw/2;

W = fft(z(n-1,:));
W = W(1:end/2);
W_total1 = [W_total1 W.'];
end