I searched and didn't find an answer. The step-by-step stated this:
Q3. What about checking the headspace after I assemble?
A3. While checking your headspace is never a bad idea, it isn't required for this operation. The headspace of an AR is set by the bolt's fit into the barrel extension. As long as you are using a good quality bolt and barrel, that was headspaced by the manufacturer, you are good to go. If you are assembling very old/used barrel/bolt, then you might want to consider checking headspace. Changing the barrel has no effect on headspace, because the barrel's seating in the upper receiver makes no difference, it is all in the bolt and barrel extension.
So my question is, if I order the parts to build an upper myself, the barrel and extension come joined and 'headspaced'? All I have to do is screw everything together?