In the past I used a service. Decent results, and they did the cleaning. They can put them online and you get to reject a certain number. Not cheap.
I've also scanned them. Slow. And it takes some prep time. And a good scanner of course.
Now I use a camera, with macro lens. Better results than all but the best services. And pretty fast, depending on how you set up to do it.
I've used an old bellows slide copying rig from the film days. Works well. You can find them cheap on eBay.
I've also used the same setup I use for negatives. I have a slide holder from the scanner I used to use (although not strictly necessary) and then I put them a bit above my iPad set with a white screen (there are apps to do a white background, or just do it with a white image at full screen). The trick is to have it a bit ABOVE the light source and use a fairly wide aperture so you don't pick up the pixels on the iPad. Focus in, set up a good speed, and start firing away. Easier if you tether of course. Works really well; a fun project actually. Helps if your tripod has a way to get above the iPad of course since it needs to be on the same plane as the slide. A lot easier than it sounds, especially if you've ever done macro work. And it helps if your camera has focus magnification in live view; focus in so you are focused on the film grain. I now prefer this method.
Nikon sells an add-on that fits on a lens, but it's not necessary. YouTube is full of how-tos. Here's one:
And BTW, if you do a lot of color negatives, look into Negative Lab Pro. Not cheap, but it really is amazing and fastest way to post process color negative film. It's amazing.