1) Felt the board roof deck with 30 lb felt, then chalk a line marking the top of each course of slate.
2) Attach your cant strip, nail your starter row, and begin slating.
3) Periodically leave a slate out and install a roof jack.
4) Proceed in this manner until you have staged the roof with adequate roof scaffolding.
5) When done, install a slate hook where ever a slate was left out. Hook a slate in place. The slate can always be removed again later should there be an unlikely need for future roof scaffolding.