Various methods have been detailed in the relevant study guides.
A method is planned so the prediction can be tested.
It is sometimes appropriate to setup a control experiment.
A control experiment is necessary for comparison purposes and proves that any change in the results is due to the independent variable being altered.
A method needs to produce accurate, reliable results.
The accuracy of a measurement is dependent on the quality of the measuring apparatus and the skill of the scientist involved.
For data to be considered reliable, any variation in values must be small.
Repeating a scientific investigation makes it more reliable.
An appropriate control experiment would be to replace the enzyme with water at 40°C.
This will allow comparisons to be made and prove it is the changing temperature affecting the breakdown of starch.
Accuracy can be improved by using a syringe to measure liquids rather than a measuring cylinder.
Reliability can be improved by completing the experiment at each temperature more than once and calculating an average.