How to Assign Static IP Address to iPad?
Firstly you need to do the assigning via your Router. Normally by reserving an IP address within your IP range for a device. Using its MAC address.

*Without knowing what your Router Model is no one is going to be able to tell you in accurate detail. There is also the issue of being able to do this if you router is configured by your provider and if they give you the options to do this or by pass they standard idiot proof settings and go to Advance settings.

Please provide more info.*

To get you wifi Mac address from your iPad.

Go to Settings->General->About.

There you will see the Wi-Fi Address which is the MAC address.