The Blanket and Plastic Sheet

The blanket and plastic sheet go back on when you are confident you have no leaks.  The insulating blanket orients so the passageway fasteners are clear for the bolts to go up into the car's floor.  The plastic sheet has the same holes to help orient it.  Drop some of the fastener bolts down the passageway fasteners from the top temporarily to help locate the blanket and sheet while you work on the butyl tape.  Get everything sealed up and make sure the holes in the blanket and plastic sheet line up with the passageways.

As you can see, this is never going to be a perfect water-tight seal.  Nobody should ever drive their Tesla through standing water that would flood over the top of the battery pack.  If this blanket became soaked with water, it would quickly mildew and eventually begin to corrode the metal lid even if no liquid water got down into the pack.  Moisture would quickly begin to condense inside the lid with a cold/wet blanket on top of the warm, dry battery.  If you car is flooded like this and keeps running, you will be best served to drop the pack and dry everything out manually even if you think the inside of the pack stayed dry.

Hot and Cold - If you are trying to re-use the original butyl tape, it will be best to have it as warm as possible when you try to install it.  The same goes for the roll of tape from Amazon.  If you are replacing it, keep that inside at room temperature if the garage is cold.  If you have butyl tape on the plastic sheet and want to remove it, place the sheet in the freezer for a bit and tape will come off easily.