a. You can't carry out a scientific experiment just like that. You need to use the proper method.
b. You can't carry out a scientific experiment in any old manner. You need to use the proper method.
I looked up 'just like that and it seems to mean 'unexpectedly' and 'suddenly'.
The idea I want to express has got nothing to do with suddenness. The idea is that there is a proper way to do something. You can't go about it haphazardly.
I wondered if 'just like that' could have that meaning here. I suppose (b) does say what I want to say, but I am not sure it is natural.
You don't carry out a scientific experiment in a happy-go-lucky manner. You proceed according to a method. Not all methods are good.