You can access the organise tools in various places.

The organise tool allows you to save one or more Inbox conversations or individual items to a contact, save them to a different contact and/or give them a label.

When composing a new item

In the bulk action controls

Activated when you select one or more conversations in Inbox.

In the conversation panel

Activated when you bring up the conversation panel by clicking on a conversation in Inbox or a contact's activity feed.

On an individual item in the conversation panel

Click on the ellipsis on the item to find the organise option.