Add to this answer:This is a subject that over the years has probably raised more questions than answers. Firstly make sure that you negatives are sharp! It is a fact that more images are spoiled by poor handling of the camera than bad focussing or exposure. Use a tripod wherever you can and when you cannot, use a shutter speed with a value of more than the focal length of the lens. E.G. With a 50mm lens the shutter speed should be no less than 1/60th. With a 100mm lens, not less than 1/125th.
In the enlarger with 35mm there are usually glassless carriers. These are crap! Single sided glass carriers are the best.
When you focus with a negative in the enlarger using a glassless carrier and get it bitingly sharp by using a grain focussing device. Then go away to get the piece of paper by the time you are ready to start, the residual heat from the lamp will have warped the film and knocked it out of focus. This is one of the facts of life. The warp is always upwards so the single sided glass carrier stops this
If you cannot get a single or double sided carrier,the easiest way around it is to focus the negative, leave for a couple of minutes with the light on whilst you get the paper ready and then re-focus. Once you are happy turn the lamp off and put your paper in the frame and expose normally so the negative doesn't have a chance to cool and warp back to the original shape..
If you are using Kodak paper, stick with it whilst you can. FUJI is on a very thin base and will crease easily. I find that I cannot get the depth of colour from FUJI paper as I can with Kodak
1. use mirror lockup
2. photograph at the prime f-stop opening of the lens