How to hold rear derailleur sprocket arm extended
Assuming you've trimmed the chain to the right number of links already...

First, put the gearing in small-small, whatever that is.

Thread the rear first.  Use a short length of old spoke through the end of the chain to stop it pulling through.

Then thread the front.

Pull both ends together at the bottom of the chain-path and use a double hook of old spoke to hold them together with a small bulge of slack chain is upwards at the joint.

Stop here, and check that your rear chain-path is correct.  The little tongue in the middle of the cage is a common culprit.  If not right, rethread.

Do up your master link or pin according to instructions.  Then check for a stiff link at the joint.