I organized a seminar. The first step was getting funding, the mechanism for internal funding will vary by institution. Then I simply sent out invitations to speakers who I knew could speak on the topics I wanted for the seminar. After that it was a matter of organizing the various things with some admin support for sorting out payment of conference fees and registration (and afterwards payment of speakers' expenses). Publicity, food & drink, CPD points where relevant, papers from the seminar, materials for the delegates, directions to the venue including posters around campus.