Hi Nishad,
If you are going to classify Landsat Data, you should do a Supervised Classification. This means that you provide input points, whose Landuse you know. This is an iterative method, where you need to provide inputs for the algorithm to give a better output.
The Link that you shared does Un-Supervised Classification, i.e. without any apriori knowledge, and hence the output quality will be lower.
I'll suggest you have a look at the
Semi-Automatic Classification Plugin for QGIS. The website has excellent video tutorials which show you how you can achieve a decent LULC classification.