Following are the steps to perform Hierarchical Clustering involved in agglomerative clustering:

At the starting point, treat each data point as one cluster. Therefore, the number of clusters at the start will be K, while K is an integer representing the number of data points.

Form one cluster by joining the two closest data points resulting in K-1 clusters.

Form more clusters by joining the two closest clusters resulting in K-2 clusters.

Now just repeat the above three steps until one big cluster is formed.

Once a single cluster is formed, dendrograms are used to divide into multiple clusters depending upon the problem. We will study the concept of dendrogram in detail in an upcoming section.