First you need to keep in mind which enzymes you wish to study. Secondly, select the plant tissue say leaves, seeds, flowers, stem or roots which may possesses the enzyme of interest. The plant or plant parts may show seasonal variations, too. Once you have identified an enzyme, prepare homogenate of plant tissue and assay it for the presence of the enzyme of interest.
Enzymes generally show substrate specificity. If a particular enzyme is isolated from a plant and exposed to a variety of substrates, the specific function of the enzyme would be found. Alternatively, biochemical mutants have been used frequently to study or decipher enzyme functions and is still very relevant. Many articles exist on internet for the study. Please go through theseArticle A procedure for the in vivo determination of enzyme activity...