23s, huh? Give me about an hour and lemme see what I can dig up for you.
In general, though, Koop did mention a rather important facet of improving your pop'n play - yes, please get used to using your fingers! I'm not sure whether or not you've mastered just lightly tapping the buttons (machines in Japan have crazy-sensitive buttons!), but both that and using your fingers to do so are pretty important to get used to.
The rest I'm going to copy and paste from an earlier post I made and tailor to your situation:
==
If you don't already do so, start using the two green buttons as your home position (unless you know what the first notes of your song are going to be
).
Play for long stretches of time (4+ hours), if possible. In your case, I'd also recommend finding an arcade that's rather empty, so that you get the most playtime. Also, if you haven't already,
purchase and make use of an e-AMUSEMENT card to help track your clears/scores.
Set a goal for yourself (like clearing or fever clearing - that is, clearing with a full bar - a particular song;
don't try to aim for higher scores or Extra Stages until you've absolutely mastered the basics!) and try to complete that goal on your final stage, every time. Normas (the conditions that you set up on Challenge Mode to get points) are good for this.
Before you start 20s, it's necessary that
you should absolutely know button placement. Levels below that give you an opportunity to learn it. In fact, if you haven't already tried Lesson [N], you might want to take a look at it (it's in pop'n 5, or search for レッソン in namesort).
Lesson [N] and
Lesson [H] are
great tools for learning patterns at 15 and 30 respectively.
Use Hi-speed.
Try to find a BPM that's right for you, use mental math to find the right hi-speed for each song, and
gradually increase your scroll speed tolerance to about 600~700 by the time you hit 30s.
You have access to Party (and later this month The Movie - I'm jealous
);
play Expert Mode. There is a feature in Party called the
Random Level Course, in which you pick a level and the game generates a course based on that level and the three levels below that. This is a really good way to practice charts above your level without fear of losing your credit.
On that same token,
use your first and last stages to practice songs that you're unsure of and/or you want to improve on. Don't keep on playing songs you know to develop your sightreading skill.
Using random song select is a great way to both increase your sightreading skill while potentially picking up a new favorite; there should be one for lv21-24, if I remember correctly.
Once again, lemme get back to you on them 23s
edit Okay, just sat down to review charts and realized that there are way too many 23s to do, so it's going to take me a while to find charts that look like they're good for practice.
new to the Pop'n Game 
