Stem cells or bone marrow are donated by another person (a donor).
To prepare the patient for a transplant, he may be given chemotherapy, sometimes along with radiation therapy. This is called a preparative regimen or a conditioning regimen. The standard transplant uses very strong treatment that destroys the diseased cells. It also destroys the immune system so it will not attack the donated cells.
After the preparative regimen, blood-forming cells (donated bone marrow) are given (infused) through a tube or central line that goes into a vein in the chest. The transplanted cells move into the spaces inside the bones where they create new marrow. They grow and make healthy new red blood cells, white blood cells and platelets.
The process is all carried out under sterile conditions as there is a serious risk of infection to the patient. The actual transplant looks very similar to a blood transfusion, with the bone marrow being transfused instead of blood.
There are plenty of websites that go into more detail if you want to find out more. Just enter "bone marrow transplant" into any search engine and lose the next couple of hours!