I want to redo my cupboard

You'll need to sand them first before starting so that the paint will adhere to them. After that you'll need to wash them so that they are clean from all dust and debris. Here is Better Homes and Garden article that might be helpful as well... http://www.bhg.com/kitchen/cabinets/makeovers/kitchen-cabinet-paint/, good luck

You could use chalk paint on them, which would require much less prep work. You would still need to lightly sand them, but the chalk paint is made to stick even over a shiny/veneer surface and cover it.

Lightly Sand then wipe all dust off... and if you are painting them use brush or Roller.... let dry , lightly sand and wipe again and give it another coat..

I like to take the doors off, use tape on back to number them, take hardware and hinges off. I lay them out on saw horses, or boards on buckets in backyard or garage. Then I sand, you can use liquid sandpaper too but I haven't done this. Then I use a brush to do edges, and a foam 6" roller, and drag off with a good badger brush. I start on back of door, dry and flip to do fronts last, I like 2 coats. If there is a recessed design I sometimes, put a distressed finish on to make the design show.

If you are going to do hardware change and the holes don't line up, fill with wood filler first, sand, then paint. Make a jig, so you can quickly drill holes for new hardware and its the same on each door.