Let's see if I can help. First of all. Be sure to read Vincom's thread about how to install properly a
Custom Recovery. Then, go to your custom recovery. Which most likely will be TWRP. I suggest you to Mount the USB Storage and place the CWM zip and your wanted custom rom zip with gapps into your storage. Install CWM and then reboot to recovery. It might take a little bit till CWM starts. Once you are in, delete Cache and Dalvik. Then, install the custom rom. Once you are done, you are done. Before installing gapps I'd suggest you to boot it so you make sure it's all working properly. Last time I installed at the same time (the ROM and Gapps) I saw so many pop-ups that my phone got stuck! Make sure you are using a stable version. As seen several times, nightlies often come with often more trouble than a stable version. They are nightlies for some reason! Right?
I hope it works!