where exactly on the door were the Israelites to put the blood?
The Israelites were to apply the blood to the two side posts and to the lintel, the top of the door. The instruction was not merely to put blood on the door generally; it specified each side post and the door top.
Les teaches that the Israelites in Egypt were to apply the blood to the two side posts and to the lintel, the top of the door. He notes that no Jew in Egypt understood what was happening, but God already had the final picture in mind. God was drawing an outline of the cross, because He did not say merely to put the blood on the door; the implicit instruction was on each side post and the door top, or lintel.
More Questions
did the Israelites understand the meaning of the blood on the doorposts at the time?
No Jew in Egypt understood what was going on when the blood was put on the doorposts, even though God already had the final picture in mind.
What was God picturing when He told Israel to put blood on the doorposts?
God had the final picture in mind when He gave the instructions for the blood: He was drawing an outline of the cross through the specific application to the side posts and lintel.