Start by checking the terms of your lease. The lease may prohibit subletting altogether or may contain restrictions: for example, only allowing you to sublet the premises as a whole.
Where subletting is allowed, the lease will generally state that the landlord’s consent is required (and you will usually be required to pay your landlord’s legal costs). You should take legal advice on the best way to approach the landlord.
You will also need to ask your lawyer to prepare a sublease covering the agreement between yourself and the new subtenant.