To add a Custom Field navigate to Settings > Account Settings > Client Settings and click Manager Custom Fields, then click New Custom Field in the top right corner.

In the field Custom Field Name add the name of the field you want to create. You can add an optional description, this will create a tool tip you can hove rover on the client file to read a description of this field. You can select the section the new field will be held in on the client file and what Type of field it should be. then click Save Custom Field to create the field

If you navigate to a client file you'll now be able to see your new field

You can now filter for data in this field on your Client List. you can click Columns to add it as a default column on your Client List view

Or click Add Filter and search for this field to filter for data in this field

If you wish to add data from this field to an email that will be sent to your client navigate to Settings > Template and either create a new template or edit an existing one. On the right you'll see a section called Client Variable, click this section to expand it. Here you will find the Client Variable list, find the section you added your new field to, then click this section to expand it

You can add the newly created variable to an email template to automatically populate the data held in this field in an email