Patience, ADRM, Patience.
First of all, the remaining structures on the site have to be demolished and removed. Then comes the shoring - a site this size will take some time. Given the size of the site though, once the shoring is done at one end, or the other, excavation can probably start while the shoring continues in other areas. From what I saw before leaving Toronto for a couple of weeks, all the equipment on site was configured for the demolition activities, not excavation - with claws instead of excavation scoops.
Initial shoring has already been done around the parking lot in the south-east corner of the site, where the Toyota on Front building used to be, and the Globe initially planned to build their new building. I do not know if the shoring that is currently in place is to the depth required for the new The Well plans, or whether the existing shoring would need to be redone, or somehow otherwise augmented to get it to the required depth.