Ratha SaptamI is for invoking Sun God.
TarpaNam to SrI BhIshma has to be offered on BhIshmAshtamI which is normally the day next to ratha saptamI.
Procedure:
(a)On BhIshmAShtamI day morning, perform sandhyA vandanAdi nitya karmas.
(b)Be seated facing East .
(c) Say the sankalpa " BhIshmAshtamI puNyakAle BhIshma TarpaNam karishye"
(d) Having YajnopavIta in NivIta mode, offer Arghyam by both hands in pure water without mixing tila by chanting the following Slokas:
1. bhIshmah SAntanavo vIrah satyavAdI jitendriyah !
AbhiradbhiravApnotu putrapautrocitAm kriYAm !!
bhIshmAya namah idamarghyam !
2. vaiyAghrapAda gotrAya sAnkritya pravarAya ca !
aputrAya dadAmyarghyam salilam bhIshma varmaNe !!
bhIshmAya namah idamarghyam !
3. gangAputrAya SAntAya Santanoh AtmajAya ca !
aputrAya dadAmyarghyam salilam bhIshma varmaNe !!
bhIshmAya namah idamarghyam !
anena arghyapradAnena bhIshmah PrIyatAm !
In fact all BrahmaNas should necessarily offer their respects to SrI BhIshma pitAmaha. In my humble view, he alone deserves the first praise of honour by all vipras as he considers them dearer than his own father.
My respectful salutations to SrI BhIshma.