All great advice, and what I will add will be more of what was already said, but here goes.
I learned to shift my old Buick, from years as a young passenger, observing and studying my Grandfather and my Father and the way they shifted their Buicks. My Grandfather, my Mother's father, shifted the car just like my own Father did, so it's not like my Father learned from his own Father how to do it.
Never once, did I observe, either one of them try to "get out into traffic" or "get up to speed". The car gets up to speed, slowly. There is no revving, flooring it, racing it in each gear, to get ahead of anything, in my experience.
My Dad, used to start in 2nd gear. That produced more slipping and at times, he would stall the engine, but it worked. My Grandfather always started from 1st gear, and that's what I do as well.
I feel, all Buicks are different and need to be learned. My 27 Standard is much different than the 27 Master, and I remember the 24 6 cylinder was different yet again (but I haven't driven it in over 25 years).
I never have to double clutch when upshifting because I wait for the rpms to drop, as I was taught. I avoid downshifting and would double clutch from 3rd to 2nd before a turn into a grade for example, but only at a road speed of less than 5mph. If I needed to get to 1st again, I would stop and start out again.
When coming to a stop, leave it in 3rd, come to a complete stop, and shift quickly through neutral to 1st again and leave it there until ready to move. Don't put it in neutral and let out the clutch at a stop, if at all possible. Getting back into gear, especially when warm, can be a grind fest. There were rare occasions when my Dad would have to shut the engine off, put it in 1st, and start the engine with the clutch depressed, but looking back I think that was more of a factor of a low level of transmission oil and those gears were spinning too fast.
The heavy gear oil works for me. Hugh needs a different mix for his car, all Buicks are different! When I first start out, The Master will shift quietly, the Standard will always, always be a bear to get into 2nd, but after the first time, it's fine. After both car transmissions are "warm" The Master always "snicks" just a little in every gear change, the Standard will shift mostly silently. Of course, that is if I do it right, if I don't let the rpms drop or my timing is off, all bets are off! It's all part of the fun! It's never a boring ride like in a modern car, I love it!
Your Roadster is beautiful! Post more pictures of it please, and let us know how you make out and if you (hopefully) have better luck with your shifting!