I'm going to start by being honest with you. I really wish I could offer a quick and easy way to be great at this technique. I can't. This is a skill that truly must be practiced. I am going to show some great tools to help but there is no way to get great without practice.
I recommend practicing with YOUR buttercream. I know some people recommend using toothpaste or mashed potatoes. That will only make you great at working with those. You wouldn't practice the flute to be great at the piano! Buttercream isn't especially expensive and can be reused over and over.
One of the trickiest and most important steps is ensuring your buttercream is the correct consistency. It needs to be thin enough to pipe a straight line without breaking. Sometimes, you'll want it even thinner than that. Roses and most buttercream flowers need to be right at the smooth line. Drop flowers and others will need to be even thinner. Play with your buttercream. Try a few different consistencies and get the feel for them.
When you are piping, you will want to steady your hand. Sometimes I rest my hand on the edge of the stand or table. Other times, I use my left hand to steady my right. In a real pinch, I'll rest my elbow on my waist. The key is to prevent your hand from shaking.
If you do make a mistake, you can use a paint brush to gently guide the buttercream to the right place. If you are piping onto buttercream, use a bit of water. If you are piping onto fondant, you will want to use alcohol so it evaporates and doesn't discolor the fondant. I also use paintbrushes to add luster and color to piped details.
In this video, I share a few tips of how I pipe, use the paintbrush, and make some simple buttercream flowers: https://youtu.be/n_1CYyrbktc