First, you need to know which view of the component you need to override.
In our example, we will override the view 'list' of the AdsManager component.
Then, in the root of your Joomla! website, go to the 'templates' folder and inside your template folder.
In our example, we use the default template of Joomla 3.0.
In the template directory, it's the 'html' folder who will conteint the override. Inside it, you need to create a directory named like the component folder.
In our example, we must name the folder 'com_adsmanager'.
All the override for every view you want for this component will be created in this directory. For example, to override the 'list' view, you need to create a 'list' directory and create, in this folder, a php file named after the one you want override.
To avoid any mistake, we advise you to copy-paste the php file you want override and then update it like you want.