If you find a slime transformation serum in a store somewhere, then chances are, that it will have a built-in mechanism that makes it so that one’s body will always subconsciously try to retain a shape that is close to the user’s original shape. This is a great feature if you’re a beginner and you just want to get a feel for what it’s like to be liquid, while not having to expend conscious effort to remain recognizable.
But it is also pretty restrictive. Luckily, there are also so-called "shapeshifter's" slime transformation, which allows the user to override the shape-correcting mechanism and take manual control of the flow of liquid in one’s body, just as if every part of one’s body was an individual muscle. Which sounds very simple when described like that, but it does take some getting used to. After all, when you go from being a solid to a liquid, you go from having a few hundred degrees of freedom, to having practically tens of thousands of them, which can be overwhelming. Also, being a fluid allows you to not only expand and contract your "muscles", but also to flow, which can feel very alien to someone who hasn't tried it before.
There are also serums that give the ability to control other properties like color, transparency, viscosity, and, if you want something extravagant: bioluminescence.
You will also find serums that provide NO shape correcting mechanism whatsoever. These are not at all easy to use, but they do make for excellent entertainment in a game of charades, and a means for skilled shapeshifters to show off.