Neonates require warmth. And just after they are born, they are wet and the slightest little draft of cool air can chill them. Momcat will groom them, but it normally takes about an hour or so for them to become completely dry. So for that first hour after birth, it is critical that they stay warm.
Normally, Mom is still busy delivering and freeing the kittens from the sac as they are born and she may also eat the placenta. All that takes time. So the first, second and maybe even third kitten in a litter has to standby a while until Mom can devote her attention to them. Make certain they are close to her body but out of the way of the arrival of the rest of the litter. Don't put them up next to Mom's head as she may choose to attend to them instead of attending to cleaning up kittens being born. I use torn-up old bed sheets in my nesting box, so I take one of these, double it up and drape it over my girl's back then gently place the waiting babies up next to her - the sheet provides a shield from the babies putting all that wetness onto Mom's fur where it is hard for her to clean herself, but allows her body warmth to come through the thin fabric. You can also drape a few of the pieces of bed sheets around and over the babies so they can keep the warmth in. Of course, have other thick padding that can fold and drape easily in the nest as well as Mom will want to burrow down to ensure that warmth.
Never, ever place a birthing nest on a concrete or uninsulated floor surface. A lot of houses don't have insulation under the sub-flooring of closets (shoddy business if you ask me!) so I don't like closets. I also find them hard to get to in case I need quick access. But, if you ~must~ have a nest on an uninsulated floor, place a heating pad under the nest but only so it goes halfway and does not cover the entire bottom of the nest. If only half of the nest is heated, then if it gets too warm, Mom will move herself and her babies away from it so as not to cause damage.
Hope this helps,