As has been mentioned I believe the old rule was that the Blessed Sacrament was incensed with three double swings of the thurible. The rule for single/double swings is no longer in the GIRM (England & Wales and Scotland), but the rule of using three swings still is, so in practice this has become three triples (i.e., up, swing, swing, swing, down three times over).
As to when the Blessed Sacrament should be incensed during the procession I don’t think there is any rule. As soon as the monstrance is collected (from the tabernacle/altar) would be standard, perhaps the priest would do this before recieving the humeral veil if the monstrance was on the altar. Also, if the monstrance is put down at any time (for adoration, or before reposition), but again the priest would do this. If there are stopping points along the route for scripture readings, etc, then they would be logical places too.
Any suggestion of walking backwards, in an alb, with a lit thurible, over potentially uneven ground, whilst constantly incensing a moving monstrance is just asking for trouble in my opinion.
As usual, the best answer is ask the priest what he wants you to do; good organisation (and preferably practice) before any liturgical event adds to the sense of reverence - anything out of line breaks the meditative prayer of those watching.
All the best
Martin
