Legends of Runeterra is a brand new entry in the digital collectible card game (CCG) arena, set in the League of Legends world. Available on iOS, Android, and PC, the game is made by Riot Games. The name will be familiar to League of Legends and Valorant.
This guide will cover how you can play with friends on any platform with relative ease.
Pre-Requisites
The easiest way to ensure you can play with each other is to make a complimentary Riot ID by visiting the official games website. Once you have made, its best to sign in to the game using that.
Getting to the guide
The steps are fairly simple. Once you have signed in to the game with a Riot ID, you will see the main menu.
At the top right you will see the cloak icon, to next to the two cog icons. Tap on this.
Hover your finger over the circular portrait to find out your Riot ID. This will appear as UserName 1234.
You can share this username with a friend or ask your friend to share theirs following the same steps.
Once you hav a friends Riot ID, tap on the small lock with a + icon on the far right. This will bring up the Add a Friend box.
Input your friends Riot ID with number in the box and submit Add Friend.
When your friend request has been accepted, you will see them in the LOR section. Tap on their name.
A new box will open on the left that will allow you to chat. In this new box will be a Challenge button at the top right. Tap on this.
Choose the type of match you want to have with your friend.
Wait for your friend to accept the request. 
Now you both will have to choose the decks you wish to use for the upcoming battle.
Once you get past the loading screen and see the board, it means you are now able to play against each other!
The following methods works with playing across the same platform and friends on other platforms.
