The institutions that manage your financial assets - bank, mutual fund, brokerage firm, managing partner in the case of limited partnerships, insurance company etc - have identified you as the legal owner of the assets under their management or control.
The legal owner on the institution's records must be officially changed to reflect your trust. You must contact the managing institution directly to change the legal owner. Each institution will have its own forms and process that they require you to follow to change the ownership.
A change of ownership notification letter requests that the managing institution initiate whatever is necessary to facilitate the change of ownership from you as an individual to you as trustee of a living trust.
Some attorneys like to have an asset assignment form signed as well to ensure that the asset can be placed into the trust by the probate court if by chance the client was to die before the institution officially changed title.