Hi Hlybbi,
1. You need to create your custom toolbar template first
Open file "\richtexteditor\scripts\config.js", you will find the toolbar section code below.
toolbars :
{
},
Add your own toolbar template in it, like
toolbars :
{
"mytool": "{bold,italic,underline,justifyleft,justifycenter,justifyright}",
},
2. Set the custom toolbar for the editor
<RTE:Editor ID="Editor1" runat="server" Toolbar="mytool" />
More details please refer to http://richtexteditor.com/document/, the "Toolbar Customization" section.
Regards,
Ken