how to make a custom field mandatory?

Hi,
Im using zen-cart 1.3.9b and i have created a custom template (called sdmc if it helps to name any file paths).

I have tried searching but found no answer (or atleast one i can properly understand).

I have created a custom field on my create account page and i would like to know how to make this field mandatory.
The instructions i used to create this field are located here

I have tried looking at the code to try and figure it out by myself but i dont get anywhere!

Any help would be great.

Thanks

Al,
I would first use the admin panel/ tools/ developers tool kit

1. search for:
2. telephone number
3. This is not an easy task because you will notice all the .php files associated that are needed.

sorry for the delay in reply, ive been a bit busy at work recently.
I have done this haredo and i only get a couple of results, none of which help me.
I believe ENTRY_TELEPHONE_MIN_LENGTH would help me but i cant find where the minimum length is defined.