Hi there old72er

Just to elaborate a little on what Chipper said...

While in NEUTRAL with the clutch ENGAGED, you have to match the engine rpm to what it would be if you were in the gear you are down shifting to, with the car at it's present forward speed
Then push in (release) the clutch and QUICKLY shift into gear with no hesitation.

If you hesitate with the clutch released, it gives the input and countershafts time to slow down and de-synchronize.
With this in mind, a thicker oil can actually make down shifting more difficult.

To make it easy...
Let the car coast down to idle speed in high gear.
Dis-engage the clutch and shift into neutral.
Engage the clutch and bring the engine up to the rpm you would have been at when you up-shifted into high.
Dis-engage the clutch and immediately shift into second.

You may get a little grinding until you learn to fine-tune your shift, but it shouldn't be much.

Hope that helps. 🙂

PS
Heavy oil helps on the up shift of course.
In this case there is no need to double-clutch.
Just shift into neutral, wait the appropriate amount of time for the input and countershafts to slow down the appropriate amount due to oil drag, and up-shift.