Card Fusing Tutorial Anyone?

I've been looking for clear answers on building up your cards. I know it goes from Common (1 star) -> Uncommon (2 stars) -> Rare (3 stars) -> Special Rare (SR 4 stars) -> Super Special Rare (SSR 5 stars) -> Ultimate Rare (UR 6 stars) -> Legendary (7 stars)

I have an Ultimate Rare Black Widow from using the Rare Black Widow cards from referrals. I know I had to do 2 Rares with Star open (2 of 3 stars lit) to make the SR card. I did this twice and then put 2 SR cards together for the SSR card with Star open (4 of 5 stars lit). I can't remember if I had to have 2 SSR cards when I created the UR card.

My questions are these:

Why doesn't this seem to work with other cards? I can't just keep getting rare versions of other cards and put them together like I did the Black Widow cards.

How do you turn a UR card into a Legendary card? It says only available through fusing.

Any ideas?