There are basically two ways to do this. The first is to contact someone you know who does detecting or do it yourself. This could result in a chest full of coins or old relics from the past which can be sold on Ebay or you could keep them. There is a lot of luck needed for this and it could produce no results. If you have someone else scan the ground for you, they will expect a share of the find. Usually it is a 50/50 split.
The other way to do it is something you can do time and time again. There are clubs of metal detector enthusiasts who want to perfect their technique and who just like to get out in the fresh air for a day.
Speaking to a coordinator will help but essentially what you do is bury metal objects for them to find. It is a good idea to make a map of where you placed these objects to retrieve what isn't found.
The detectors will scan and dig in your field. Most of them are very good about replacing the soil after their digging but it is worth giving them a gentle reminder to do so.
They obviously will pay you or the coordinator of this event.
The profit of this can be increased by supplying food and drinks. This can either be at your farm or even from the back of a pick-up. Light refreshments such as sandwiches and soft drinks is enough. If it is cold weather, opt for coffee or soup in a thermos. People will pay to have a day out like this.
If you live in an area where health and safety concerns are strictly enforced, be sure to seek advice from your local authority as to what if anything needs to be done to show you have shown due diligence.