Unhappy [Q] How do I Customise My MX Android TV Firmware (Zip.File)

Hi guys im a noob to the forums and to rom customisation in general so im hoping someone can help me.

I have a generic MX dual core, and have been supplied a flash firmware and image from the supplier. This image is basic and does not contain all of the repos that i want, nor the apps that I require.

The image supplied is a zipfile and contains:

factory_update_param.aml
hd18d_mx4.zip
recovery.img

I am able to successfully restore my box with this image (so I know that the image is working fine).

Within the hd18d_mx4.zip there is:

Directotories: data, META-INF, recovery, system
files: boot.img, bootloaded.img, logo.img

I can see the all the system APK files within the system/app folder. The problem is when i try to add another APK file into this directory within the archive (in this case videomix.apk), I am not able to restore the image to the MX device.

Can anyone help me with a noobies guide to do this. I have read that the apk file has to be signed, and the zip file has to be signed also?? But im not quite sure what this means, nor how I go about doing it.

Any help or tutorials will be much appreciate. I have downloaded the APK-multi-tool but dont have a clue what I need to do

Look forward to your replies.

P.S. Im using a windows 64 bit machine and not Linux (if that makes any difference).