sTeam Achievements relies on you making your achievements public and signing in using either your 'Custom URL' or 'Steam ID'. Steam uses a number of names to represent you which can cause confusion, hence the attempt to explain what these are below:
Open your favourite browser and go to https://steamcommunity.com/
This is your Steam Username, typically no-one else will see this name, it’s private to you. This is NOT required by sTeam Achievements. Login.
Locate the Actions section and select ‘Edit my Profile’
Now we’ve arrived at the page that best explains the majority of the names available in Steam:
The last step for allowing access to your details from sTeam Achievements is to make your achievements publically available.
So hopefully you now have enough details to make use of sTeam Achievements. It’s also worth pointing out that if you want to create a team with your friends then they’ll have to configure their profile using the same steps.
Hope this helps, happy gaming!