I personally would recommend using the small boost bar, that one fills up towards the left of the bar, making it in my opinion way easier to look at during the race since you don't have to look all the way into the bottom right of the screen to see if it's filled up. It also uses very a distinct red and green color that makes it easier to see out of the corner of your eye if you don't want to take your eyes of the kart. I also don't like how it somewhat misrepresents how the boost bar actually works so it might cause some bad habits for newer players, as just going for perfects isn't necessarily the optimal way to play.
Started adventure mode and the tip to look for black smoke is WAY more discernible mid race than flowing wheels. Was able to get a handful of turbo stacks. I still haven't gotten a hang in determining my drift angle reliably. Also, I'm not clear in how the reserve meter works.
The most important thing to know about the boost bar is that your goal is to hit the button as late as possible in the bar. The drift boost has two properties you want to keep track of, the speed and the reserves (aka the time the boost will last).
- The speed part is simple, a drift boost gives you a lv1 turbo, the third drift boost in a chain gives you lv2 turbo, any turbo pad gives you lv3 turbo and very specific turbo pads can give you even higher lv4 or lv5 "blue fire" turbo. The flame out of the exhaust is the main indication which lv of turbo is active. All in all, what this comes down to is for you to want to keep drifting to keep the highest lv of turbo going. That's where reserves kick in.
- The reserve system is a bit trickier since there's no actual visual indication on screen for it. Reserves is basically the time your current turbo will last, think of it as a bar that is constantly depleting itself while you're boosting and your job is to keep adding more reserves to that bar faster than the bar depletes itself to keep that boost going infinitely. The main way to fill this bar is drift boosting. During a drift boost you'll notice you can hit the button at any time while the bar is red to get the boost, however hitting it as early as the bar turns red actually gives you near 0 reserves, causing that boost flame to disappear almost immediately as the boost has no reserves to last. The real goal here is to hit the boost button as late in the boost bar as you feel comfortable doing, giving you a biggest amount of reserves. Hitting those big reserves by hitting the bar late will cause them to last into the next boost and its reserve so it starts stacking on top of eachother making it possible to keep any lv of boost going infinitely because as long as the reserves last, your current level of turbo won't downgrade.
So your goal is basically to hit the boost bar as late as possible to get the reserves filling faster than they deplete, while having reserves going hitting a turbo pad to upgrade the lv of turbo, then just continue to maintain the reserves with drift boosting so you never lose that turbo pad speed. To get the big difference between low and high reserves. Try doing a triple drift boost hitting the boost bar as soon as the bar turns red and do a triple drift boost hitting the boost bar at the very end of the bar and notice the difference in how long your boost lasts after the third boost, you'll see how much of a difference it makes.
To add on to what Wrestleman said, please keep in mind if you are using the fancy boost bar you still want to hit the bar as late as possible. That bar might give you "PERFECT"s indicating when you're hitting it in the generally good zone to keep a boost going, but you're not necessary performing perfect boosts despite the popup. Your goal should always be to hit the bar as late as possible, every single pixel in the red part of the bar counts towards the amount of reserves you'll actually get from the boost, so a late perfect is still better than a early perfect. (Another reason why I personally prefer the smaller boost bar as the fancy bar doesn't accurately represent how the system actually works)