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 - this in order to maximize the chances of the soldier in question turning out to have Psi-abilities. The next step being to equip the soldier with Psi Armor and lastly designate him/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. 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 yet again; 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.