Cover your head with a Topi (cap) sit down, pray :
بِسۡمِ اللهِ الرَّحۡمٰنِ الرَّحِيۡمِ
and then eat, Also pray :
بِسْمِ اللَّهِ وَعَلَى بَرَكَةِ اللَّهِ
and a little will suffice for a large number of people.
If you forgot Bismillah before eating, say:
بِسْمِ اللَّهِ فِيْ أَوَّلِهِ وَآخِرِهِ