This is a good question i must say and i really take friendship seriously and very stern when it comes to making friends. At a practical level in my own case, I will advise the following:
1. Do we have one thing that unites us?
In friendship, it involves two different people who are coming close together and will be sharing parts if not all of their lives. So you need a common ground of attraction that will always remind you of why you became friends and it is this common ground that will strengthen the friendship between you two. For example, i have a friend Kingsley and we are best of friends today and our common ground of attraction is music. We both love music and this was what brought us together and everytime we have problems in our relationship, it is still the factor that pulls us together again.
2. Love for one another
The two friends must love and appreciate each other in their friendship. I mean if there is no love in that friendship then it is not even friendship to start with. Both friends need to love each other and this love should be one of sacrifice that enables you to be able to always be there for the other person when he/she needs you, to make sacrifices for eàch other, feel for each other etc.
3. Sincerity
This is a very important factor in friendship. Many friends at times are too scared to tell the other partner the truth thinking it will hurt them. For example, you discover something in your friend that is not nice or maybe she dresses in a funny manner or something and instead of telling them you simply pretend to like it and when they go out there, they are laughed at and gossipped behind the scenes. You should be sincere in friendship and always tell your fiend the truth even if it hurts.
4. Trust
For every relationship to function well then there must be trust because without trust, then both parties will never experience growth in their frienship and will never even get to share pertinent things about themselves. Trust is needed in every friendship and it is very important.
These are the few basic and most important things in friendship.