print heading on blank cards
A primary function of Libreoffice writer is to format material and send it to a print driver which then probably sends it to a printer. A typical post card is a special case of ‘card stock’ (thick paper). Under the Format menu is a page menu where you can specify details such as size of page. You probably need to tell the printer a special ‘Custom’ size paper is needed.
Since post cards typically cost a lot more money than simple printer paper, consider cutting down some sheets of paper to ‘post card’ size then run some test samples until you are satisfied with the result.