Default value in field

Dear All,

In vendor master I want to put default valie X in field "Chk Doble invoice" (i,e, LFB1-REPRF).

Is it possible thru creating screen variant.

I don't know how to create screen variant and how it take effect in XK01 T-code i.e vendor master creation.

Requst you to please guide by step-by-step procedure.

Thanks in advabce,

Nirav