Before you attempt this, it's advisable that you save first. Proceed with awarding a soldier with high will (preferably 80+) 1 medal of each kind, then test the soldier for Psi abilites. The next step is to equip the soldier with Psi Armor, and lastly designate him or her as The Volunteer.
There are 5 types of medals - each type only being available in small numbers.
The medals are:
Urban Combat Badge (4)
Defender's Medal (3)
International Service Cross (2)
Council Medal of Honor (2)
Star of Terra (1)
Awarding a medal is a permanent action, thus the absolutely safest way is to save 1 medal of each type until attempting this. Take note that medals can be made available once more, however, that includes having the soldier in question killed, as well as fulfilling the specific prerequisite once more; the Star of Terra being the sole exception.
The Iron Will option, which is strongly recommended, is purchasable from the Officer Training School and helps tremendously to boost the Will when a soldier is promoted - thus the earlier it's purchased, the better.