Instant Shinesparking, often called Instant Sparking for short, refers to any case where the wind-up animation of a shinespark is skipped. Conversely, any case where the wind-up animation plays out is referred to as Slow Sparking.
The main way to instant spark is to buffer a spark out of an uninterruptable animation. Examples include but are not limited to the following:
Instant sparks can also be performed by breaking a spin jump while inputting a spark. This can be done by pressing jump at the same time as either shoot or free-aim, which will cause Samus to immediately spark in the direction the player is holding. If you still want to slow spark (i.e. for Escue quick kill), do the inputs without holding a direction.
Additionally, up and down instant sparks can be performed from a standing or neutral falling position simply by holding the direction first. No need to melee or Flash Shift, just press up/down and then B.
Instant sparks can be practiced anywhere that you can charge speed, but it's usually best to practice them where you'll be using them. If you want to practice specifically store sparks, the best place to do so is underneath Yellow TP in Dairon, as you can spark the slope repeatedly to easily practice in both directions.