When we want to add a custom button to a list toolbar we can add it by using a feature.But when we are activating the feature it will be apply to  the scope secified, so all the lists present inside the scope will have a button added to it’s toolbar. But if we want to add the button to a particular list then we need to specify the content type ID of the list.

 To get the content type id of a list we need to follow these steps:

1.   go to the list =>Settings=>List Settings=>Advance Settings=>click yes to the allow management content type
=>ok=>Now in the list setting page you will see a content type click the content type =>In the url you will see a value like

"ctype=0x010046759B602A72384BB7F4C50DEA9EE828"

This is the content type id for the list.

2. Now if you want to get it programatically then you can use this one:

oSPWeb.Lists["listname"].ContentTypes[0].Id.ToString();

 
Now we need to create the feature.xml file :

<?xml version="1.0" encoding="utf-8" ?>
         <Feature Id="{0af0c758-9ed9-43c8-aa2f-90a6fe879c9d}"
               Title="Custom Menu"
               Description="This example shows how you can implement custom menu."
               Version="1.0.0.0"    
               Scope="Web"
               Hidden="FALSE"
               xmlns="http://schemas.microsoft.com/sharepoint/">
              <ElementManifests>
                     <ElementManifest Location="Element.xml" />
              </ElementManifests>
         </Feature>

Here  id is the feature id which is a GUID.

Scope is web, which means the feature will be available to all the subsites also

Hidden is false, which means the feature will be visible in the feature list for a site (Where you can activate the feature)
Next: Create the Element.xml file:

Here we need to add a custom action .

<?xml version="1.0" encoding="utf-8" ?>
        <Elements xmlns="http://schemas.microsoft.com/sharepoint/">
              <CustomAction  RegistrationType="ContentType"   RegistrationId="0x0100F310C1F224701541BF37AFD23D67FEAC"
              Sequence="1000"  Location="ViewToolbar" Title="Test">
                    <UrlAction Url="/path to navigate"/>
              </CustomAction>
       </Elements>

Here the RegistrationId is the content type id of the list  and Url is the path to navigate when the user clicks the button.