Incense: when?

[VML]

Please can anyone remind me at which point in the preparation of the gifts/ the offertory on Maundy Thursday should the altar be incensed?
We have usually sung either 'Ubi Caritas' or 'Stay with me' across the whole offertory until the summons to 'Pray..etc. but PP wants us to sing 'Blest are you, Lord God of all creation..' at the appropriate point. If censing is done before the 'Blessed are you..' then it will be in silence, but if the incense comes after the offering we can carry on singing over it.
Thanks
V

[docmattc]

Should be in the usual place. After the preparatory prayers (Blessed are you...) and before the priest washes his hands.