Transaction screen variant for ME21N transaction

Hi All,

As per my client requirement PO price field (NETPR) should be greyed out for some business users when they are creating PO's. And price will be copied from Info record.

To accomodate this requirement we decided to go ahead with transaction screen variant for ME21N transaction. (By creating  a custom Tcode ZME21N)

I have some fair idea about creating a transaction variant but not complete one can somebody help me the steps required to create a transaction variant for the above mentioned scenario

Regards

Pavan