This is a useful link , albeit a little long-winded and convoluted, describing the background and reasons for making merit.
Merit can be accumulated in many ways. For example, by donating money or useful items to the temple, offering alms to monks, praying for the spirits of the deceased or simply by living a good life and being mindful of how you behave and interact with others. It is also just as easy to to lose merit, simply thinking ill of somebody or uttering an unkind word can cause you to lose good merit.
We recently held a merit making ceremony at our house, to pray for my mother-in-law's spirit guardian. We still perform this ceremony every year in July, even though my mother-in-law is sadly no longer with us. The ceremony includes a display of traditional Thai dancing together with offerings of food, fruit, flowers and whiskey. Everyone in the family will pray and thank the spirit for watching over us and then place incense sticks among the offerings.
Here are some photos of the celebrations.......