How to train a puppy to be in a dog crate

Some of my dogs have been in dog crate. This is because they have done some damages when I have been away. I know that this is because of several things. Some of the dogs have separation anxiety, and other have just been bored...
Some of the dogs have separation anxiety, and other have just been bored...

Well, I needed to have one my puppies in a crate... 
At first, I thought it was terrible. I felt sorry for the dog. He looked like he was in prison.
I got a crate, placed it in the livingroom with the door open and a blanket with the dog’s smell on it..  My puppy was very curious with the crate.
I began to feed him inside the crate. At first he wouldn't go inside to get his meal. I put some snacks in the crate and he took them, but he wouldn't eat in the crate. He carried his meals out one peace at the time to eat them on the floor. We worked like this for a week or so.
 Finally, he started to eat in the crate. When he was finnished with his meal, he started to relax in the crate. The crate wasn't dangerous anymore. Then we started to close the door when I went to the bathroom. He started to howl. OMG!! How was I supposed to stop that? How was I supposed to train a puppy to be in a dog crate without howling?
I twisted my brain to find an answer... I started to talk to him when he started to howl, so he could hear my voice. That worked a little bit.

After a while, I carried the crate to my bedroom so he could sleep in the crate at night. I thought that was clever because he could see me if he got afraid.
He got his meal in the crate, we went outside for a walk and I put him in the crate and closed the door.Gave him some snacks when he layed down. I talked to him a lot with very calm voice. I went to bed, and talked to him some  more. I told him what would happen next. (I thougt the puppy understood every word I was saying)Then I said goodnight and turned off the light. Talked to him some more, and I fell a sleep... Then the puppy started again... I talked and talked and talked to the dog. That was a nightmare of a night!! I didn't get any sleep at all. I have heard moms with (human) babies complain that they are only getting 3 hour sleep at night. I began to feel like a mommy.....
But after a week with no sleep and a lot of talk with the puppy, it began to help.
Next step was when I left the house. I always had a radio on so the dog didn’t think he is a alone...I just stood outside the door to listen. When I heard he started to howl, I opened the door and said "no". When he was quiet for 10 minutes or so, I went inside and gave him some snacks. This was done in a week. My puppy learned very quick.  
It is a lot of work to train a puppy to be in a dog crate, but it is worth it.