There are a couple of methods for joining a scheduled Skype for Business meeting:
Method 1a - Skype via Outlook
Open Outlook, and open your Meetings tab, located in the bottom left-hand corner of your screen. Double click on the Skype meeting in your calendar and the methods of joining the call will be shown to you. If you click Join Skype Meeting, this will automatically join the meeting through the Skype for Business app on your system, prompting you whether you want to join with audio or not. Joining through Skype for Business will give you the full Skype call experience, including screen sharing, IM and video.
Method 1b - Phone via Outlook
You can also dial into scheduled Skype meetings through a conferencing number. This should be listed in the invite along with the conferencing ID which you will be asked for when dialled. Please note this will only act as a phone call, you will not get any of the Skype features like screen sharing, IM and video like you will above.
Method 2 - Through Skype for Business app
You will also be able to see any Skype meetings you have scheduled through the Skype for Business app. Browse to the meetings button on the far-right hand side and your upcoming meetings will be shown. Any Skype meetings will be marked and will have a Join button on the right-hand side. By clicking on this, you will be taken into the meeting through the Skype for Business app and will have the full Skype experience including screen sharing, IM and video.
Method 3 - Through a mobile device
Make sure you have the Skype for Business app installed, log in and browse to the Meetings button in the top right-hand corner. Your available Skype meetings will be shown, similarly to how they are on the Windows client. Select the meeting you wish to join and you will be pulled into the meeting.