How do you write a sentence with sap?

Well, that depends just what "sap" you are referring to.

If you mean sap as in the sap of a tree, or other plant, that is a noun. You would use this as a noun as appropiate to the sentence.

"Maple syrup is made from the sap of the Sugar Maple tree."

If you mean sap as in slang for a stupid or gullible person, that is also a noun but with a different meaning. You would only use it in reference to a person.

"The clerk tried to give me a $7 dollar bill as change! What kind of sap does he think I am?"

If you mean sap as in "to drain", that is a verb.

"Walking through the Sahara Desert without a hat can sapone's strength."

If you mean that you want use sap to write withinstead of ink; then you would simply take a stick, sharpen one end, dip it in the sap, then get a piece or paper & start writing. ;P