Many couples that undergo IVF end up with a lot more embryos than they can eventually use to complete their families. When there is an excess of embryos, they can either remain frozen, be destroyed, be donated for scientific research... or adopted by another family. For many, this final choice is the most humane one, that still allows the embryo to grow into a child, in a loving family. There are several routes through which families hoping to adopt an embryo (it should be mentioned that this is technically a property transfer and not an adoption) can start looking for a match.
Often the simplest and cheapest solution is directly through your fertility clinic. Families with left-over embryos sometimes ask their reproductive endocrinologist (RE) to find homes for them. If your doctor has such a private stash, for lack of a better word, these embryos are often provided without additional cost. Then there are embryo adoption agencies that match donating couples with those hoping to get pregnant through embryo adoption. This is a more complicated procedure that can be costly as well. See the cost of embryo adoption for more information. Once you have been matched with an embryo, the embryo transfer and preparation is the same as with traditional IVF. If you are considering this option, you may also want to take a look at embryo adoption pros and cons.