Allah mentions this in the verse (interpretation of the meaning):
Allah will not punish you for what is unintentional in your oaths, but He will punish you for your deliberate oaths; for its expiation (a deliberate oath) feed ten Masaakeen (poor persons), on a scale of the average of that with which you feed your own families, or clothe them or manumit a slave. But whosoever cannot afford (that), then he should fast for three days. That is the expiation for the oaths when you have sworn. And protect your oaths (i.e. do not swear much). Thus Allah makes clear to you His Ayaat (proofs, evidence, verses, lessons, signs, revelations, etc.) that you may be grateful.” [al-Maa’idah 5:89]
Feeding 10 poor people with the same food that he feeds his family, by giving each person half a saa’ of the usual local staple, such as rice. This is equivalent to approximately 1.5kg. If rice is usually eaten with some kind of sauce then along with the rice he should give them some sauce or meat. If he gathers together 10 poor people and offers them lunch or dinner, this is sufficient.
Crisis Aid can do this on your behalf at the cost of £3 per person. For example to feed 10 people that will cost £30.