What does Atticus mean when he tells Scout "to delete the adjectives and have the facts" about Jem's strange behavior in To Kill a Mockingbird?
Great question! Atticus is actually referring to the last part of the paragraph you're referencing. Jem, displaying his new-found knowledge of all things Egyptian, tells Scout, among other things, that the Egyptians invented "toilet paper and perpetual embalming." Atticus, obviously recognizing the humorous error in this statement, instructs Scout to remove the adjectives (toilet and perpetual) and she'd have the facts. Doing so would lead Scout to the conclusion that the Egyptians invented paper (not toilet paper) and embalming (not perpetual embalming, since because the word "perpetual" means "continuing forever," such a phrase would be redundant).
Atticus's remark is just one of many in which Harper Lee points out Scout's naivety and Atticus's dry yet humorous response to it.
In order to answer this question, Jem's words deserve to be read again:
He declared Egyptians walked that way; I said if they did I didn’t see how they got anything done, but Jem said they accomplished more than the Americans ever did, they invented toilet paper and perpetual embalming, and asked where would we be today if they hadn’t?
In this context, Jem is enthralled to be in 6th grade. In particularly he loved the section on the Egyptians. He, therefore, began to even imitate the way they walked "sticking one arm in front of him and one in back of him."
When Scout questioned him about this, Jem responded by touting the great inventions of the Egyptians. This is where Atticus' wise words come in.
Atticus basically tells Scout to strip down what Jem is saying to it essential meaning. The Egyptians created some important things that help us even today. In other words, they were inventive.
Another way to look at it is by actually taking out the adjectives, as Atticus advises. If we did that, we would have: paper and embalming. The Egyptians did invent these two.
The Chinese invented paper, not Egyptians.