1. How to create Google Tag Manager account?

Sign up using this link https://tagmanager.google.com

After the sign up you will receive your GTM code

Click OK.

At the top navigation panel, you'll see your GTM-XXXXXX container number. 

Copy this code.

Navigate to your Diary -> Settings > Reservations > Widget and Portal Tracking 

Select Tag Manager and paste in your GTM container number.

Click Save.

2. How to add Google Analytics Tracking into GTM?

In the Tag Manager, go to your container.

Navigate to Tags section.

Click New

Start by naming your tag any way you want.

Select Google Analytics

Select Universal Analytics and click continue.

It will ask you to input your Tracking ID. 

To get your Tracking ID, go to analytics.google.com -> select the account you want to track -> Admin -> Tracking Info -> Tracking Code.

Copy the UA-XXXXXX-XX code and paste it into Tag Manager Tracking ID field.

Click continue.

Select All Pages.

Click Create tag.

Make sure you click Publish at the top right side in order for these changes to take effect.

If you have done everything correctly, all the pageview data should be going to Google Analytics.