I'm not a native speaker of Chinese, but here are some suggestions (red means changes and additions; green with strikethrough means deletions):
The word 如果 is a conditional "if"; it doesn't bear the alternative English meaning of "if" as "whether (or not)". You could also replace 否 with 不是.
I think the following is closer to natural (though it would be better for a native speaker to comment):
Where I wrote (something), you should put what they painted above the door.
The 來 is optional, but in this sentence it functions like "to", in its sense of purpose: "use cow blood to paint..."