how can embed on dwt decomposed image?

i've only decomposed my cover img. bt cant failing to code for embedding. i can reconstruct the img, only need to embed with watermark img.. plz hlp...

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img  = imread('home.jpg'); %input image 

img  = rgb2gray(img);      %Convert to grayscale image

figure,imshow(uint8(img)),title('Cover Image');

img  = double(img);

[Lo_D,Hi_D,Lo_R,Hi_R] = wfilters('haar'); %to Obtain fiters associated with haar

[ca,ch,cv,cd] = dwt2(img,Lo_D,Hi_D);     %Compute 2D wavelet transform

y = [ca ch;cv cd];

figure,imshow(uint8(y)),title('dwt imag');