Take a stout line and tie the lower block with a loop around the mast, just loose enough to slide along it. This knot is a bowline, but tied "inside out" since the force will be from the loop instead of the standing part.
You don't want this knot to slip, so back it up with some safety knots.
Note: this line needs to be loose enough to clear obstacles as the block is raised, but not so loose that the tackle becomes two-blocked up top when lifting the mast.
Pull on the bitter end of the tackle and send the loop and block up to the spreaders.
The line is then cleated to the base of the mast. The cleat will take the load when the mast is vertical. As the mast is tipped, the weight will shift to the loop.
Look up and inspect everything. Things should appear to be clear and running free (this inspection process should be performed throughout the entire setup).