apt-key deprecated

[Erwan David <erwan@rail.eu.org>]

Hello,
to install signal desktop client, I followed their instructions, which 
is to add their repository and, obviously their Kry.

However, I got a warning
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d 
instead (see apt-key(8)).

But how to do this ?

man apt-key says :
Note: Instead of using this command a keyring should be placed directly 
in the /etc/apt/trusted.gpg.d/ directory with a descriptive name and 
either "gpg" or "asc" as file extension.

So the question is how can I get a keyring file from the key file I 
downloaded ?