I hate to say it but I NEVER practice putting. Its just boring to me. Fortunately I have a good feel for it and while I could improve, I am happy with my results right now.
Range wise, I like to alternate between long and short clubs so I can get comfortable with the transitions. If you hit wedge, wedge, wedge, 9, 8 etc you will get in a groove with each one but on the course you don't get to do that. So I go something like 8, 5, 9, 4, P, 6, GW, 3W, LW, 5W, SW, D and just hit 3-4 shots per club.
That's not to say that you don't need to focus on one or two clubs sometimes and get them dialed in. I do this with Wedges and Driver a lot and pick a different club each session to spend extra time on.
Finally you need to play some. 9 or 6 holes is perfect. I like to go and hit on the range for 15 minutes and then walk over and play 9 holes, maybe 6 if its late or hot (course has a perfect spot to jump from 3 to 7). Tough on public course I know. This is probably the biggest benefit I get from my club membership.
Of course it all depends on where you are with your game. OP is a 20 so I would say you have plenty to work out on the range. Once you get down to low teens (at least in my experience) there are more improvements to be made on the course itself. Course Mgmt, confidence and just the general progression of a hole from tee to green (carrying that water hazard for example).