Belle didn't have a crate until she was over 3 years old; I got one purely to give her a place of her own (it's 'her bedroom'). It took next to no time to train her to it. Basically I put her food bowl in it at supper time (yes, she knows the word 'supper' and now, that means going into the crate, sitting and waiting for me to bring her food to her). I started off with putting a nice soft dog cushion in the bottom, and putting her food inside, but where she could just reach the food bowl. I started moving it to the back, then, once she was used to the idea of the crate, and only then, did I shut the door behind her.

Now, it's her 'safe' place. If I give her a treat that she wants to keep from the cat, or just wants to take a nap, or wants to keep a toy after the game is over , she goes to her crate (I get the toy out to put it away, though). I do have a dark bedsheet draped over it, to help make it feel more like a den for her. Either the command 'Go to bed' or 'supper' result in her going into the crate when I want her to, and she doesn't complain too much until she starts getting bored or thirsty (the water bowl is outside of the crate so that the cat can share it).