Here is an example that you can base your logic on, After the start stage, there is a reader that reads if the application that we are using is either connected or not, if the app is not connected you attach it, if not it ends the process.
The difference is that Launching an application Attaches it to the Object.
If the application is launched via an Object and you try to re-attach then it will throw an error. In such cases you need to check whether the application is already attached or not by doing as shown by @adnaneLaaziz.
So one way to Attach is by Launching as stated above.
If you want to explicitly Attach an application to an Object then you use a Navigate stage -> select the root element on the left hand side -> and then choose Attach in the right hand side actions dropdown.
At times parameters are needed to be given in the bottom screen like Window Title, Process Name, etc in order for the Object to correctly recognise the application.
We typically need this kind of Attach when there are multiple windows/applications being interacted with, or maybe something like a popup. Likewise there is also a Detach option.
You cannot use the inbuilt Blue Prism methods, actions and objects to interact with an external application without Attach so that is the use of it.
There is also some information about the Attach in the Foundation course pdf.