The aggregation, arrangement and bonding together of a set of components to form a membrane raft, a small (10-200 nm), heterogeneous, highly dynamic, sterol- and sphingolipid-enriched membrane domains that compartmentalizes cellular processes. [ http://www.ncbi.nlm.nih.gov/pubmed/12803918 http://www.ncbi.nlm.nih.gov/pubmed/12648772 http://www.ncbi.nlm.nih.gov/pubmed/16645198 ]