How to install iOS 11 on my ipad

You can't. There is no current device that can install iOS 11 at this point.

If you have an iPad 4 running iOS 10.3.3 then that is as far as it can go. There is no way to update it beyond 10.3.3

If you have an iPad Air or later than can install iOS 11 or later, currently only iOS 12 and 13 are actually available to install on those models since you can only install the most recent version of iOS your device supports.

See Update your iPhone, iPad, or iPod touch https://support.apple.com/en-us/HT204204 and the related compatibility guides below.

iOS 13 and iPadOS are compatible with these devices - Apple Support.  

iOS 12 is compatible with these devices.  

IOS 11 is compatible with these devices