Here is a prayer which he requested should be said for him after his death (in his statutes for the Collegiate Church at Middleham).
If the Latin is too difficult for you, you can say it in English.
Deus, cui proprium est miseréri semper et parcere: súscipe deprecatiónem nostram; ut nos, et omnes fámulos tuos, quos delictórum catena constringit, miserátio tuæ pietátis clementer absolvat.
Propiciare animæ famuli tui Ricardi.
O God, whose nature and property is ever to have mercy and to forgive: receive our humble petitions; and though we be tied and bound by the chain of our sins, yet let the pitifulness of thy great mercy loose us.
Be merciful to soul of your servant, Richard.