Start with the drawers ... if they are too wide, you will have to somehow trim/plane one or both drawer sides until the drawer width falls within the guidlines of your slides.
If they are too narrow, you will have to shim the drawer slides. If both of these are in play, then do what you have to do with a combination of shimming and trimming and take the last sentence below to heart.
With that out of the way ...
There is some wiggle room built into the slides themselves and you can generally get a sense of where things are wrong pretty quickly with just a couple of screws on each slide, and by taking advantage of the built in adjustment holes in the slides themselves.
Remove all but two screws on each side, leave one in the front, one in the back. Make sure the screws are in the MIDDLE of holes that allow the most vertical adjustment, as that is where the problem often is.
(You can usually tell quickly if the drawer slides are not lined up with each other along their length/height by closing the drawer and seeing if one side of the drawer want to be further inside the cabinet than the other, or higher on one side, or if it rocks excessively in any plane ... if so, rectify that situation first, with the front screws)
Leave the back screws just loose enough to allow the slides to move out of any bind as you close the drawer. Does it now close easier? If so, open the drawer and tighten up the back screws a bit at a time, repeating the open and close routine, until the drawer works like you want it.
If not, loosen the two front screws a bit and repeat the process, alternately tightening from front to back, side to side as you go.
If the above doesn't work at all, then chances are you've got a problem with the width of the installation. If it gets more difficult to close as you tighten the screws in the first above, you may need to shim one, or both, of the slides at the back, or vice versa.
Good luck ... and don't settle for less than perfection. The more you work at getting it "perfect", the more you learn, the more your intuition becomes finely tuned, and the quicker the process becomes for the next drawer.
IME, most of the problems with drawer slides stem from the cabinet or drawer sides, or both, not being square and that generally comes from like parts not being BATCH cut to the same size ... doing that one step can save you all the time and frustration of going through the above.