Sion Jenkins has told an Old Bailey jury of his shock and horror at finding his foster daughter dead.
Billie-Jo was found battered to death at the family home
Mr Jenkins, 48, now of Lymington, Hampshire, said he moved Billie-Jo twice to look at her but "her injuries were horrendous".
The prosecution claims he battered her over the head with an iron tent peg.
Mr Jenkins denies murdering the 13-year-old at their home in East Sussex in 1997. He is giving evidence in his second retrial this year.
He is accused of battering Billie-Jo to death as she painted patio doors at the house in Lower Park Road, Hastings.
The retrial has heard Mr Jenkins was told by a 999 operator to place Billie-Jo in the recovery position and to check if she was breathing.
Sion Jenkins has remarried since divorcing Billie-Jo's foster mother
But the prosecution say he did nothing to help her before an ambulance crew arrived.
Mr Jenkins told the court he had not been trained in first aid or resuscitation technique.
The former deputy head teacher was convicted of Billie-Jo's murder in 1998 and jailed for life.
This is his second retrial after a jury failed to agree a verdict in July.
The trial was adjourned until Wednesday.