How can I import LibreOffice Impress templates?

While in LibreOffice Impress do the following

1. Go to File > Templates > Manage Templates (or Ctrl + Shift + N
2. In the new window that opens up, click on Import in the bottom right corner

3. Choose a category in the Select from Existing Category window or create a new category below that

4. Navigate to where you have your templates stored and choose one of the .otp template files
5. Click Open and you're done

Edit: this was done on LibreOffice version 5.3.5.2 on openSUSE Leap 42.3.

I was preparing this answer when n8te's answer appeared: it is entirely correct and I up-voted it.

However, my answer was based on Ubuntu and the dialogues are slightly different: so I am submitting my answer for the benefit of users who experience different dialogues.

First, extract the files from the zip file, then start Impress.

Then from the File menu select Manage, or type Ctrl+Shift+N.

In the Template Manager dialogue box, select the Presentations tab, then click on the Import button and navigate to the diectory where you extracted the .otp files. You can several or all the template files and they will be imported together.

(I did not get the Select Category dialogue, but this may be because I haven't created any specific categories.)

You will now see all the imported templated if you click on My Templates. For some reason, My Templates was not in the drop-down folder list in the new presentation wizard, and I had to select all the templates and move them to one of the presentation folders, although oddly I could move them back and now My Templates appeared in the list. This may be a minor bug in my version 5.1.6.2 on Ubuntu 16.04.3 LTS.