what is sentence embeding and how to do sentence embedding for a sentence and how to use word embedding to create a sentence embedding?
It could be just a simple average of word vectors; words that are a member of the sentence. However, there are other methods that could work better. Please see this article:

In the paper Unsupervised Learning of Sentence Embeddings using Compositional N-Gram Features, a new model for sentence embeddings called Sent2Vec is introduced. Sent2Vec presents a simple but efficient unsupervised objective to train distributed representations of sentences. It can be thought of as an extension of FastText and word2vec (CBOW) to sentences. The sentence embedding is defined as the average of the source word embeddings of its constituent words. This model is furthermore augmented by also learning source embeddings for not only unigrams but also n-grams of words present in each sentence, and averaging the n-gram embeddings along with the words.