Thank you, Carlos! What can you do to help? The good news is mother cats are usually pretty perfect at it. But there are things you can do if the mother will let you.
Can you get a kitchen scale (calibrated in grams or ounces) and weigh the kittens once or twice a day? Steady weight gains is the best way to know they're healthy.
Make a chart so you can keep track of each kitten's weight. Ideally they should double their birth weight in the first week. In your case you don't have an exact birth weight, so make allowances for that.
What that means in daily gains: an average of about 1/4 to 1/2 ounce per day, or double-digit grams. For instance if a kitten weighs 100 grams at birth, he should gain another 100 grams during the first week, which is an average of about 100/7 or 14 grams per day.
There will be natural variation in this, so don't worry if a kitten occasionally gains less or even loses a little bit from one weighing to the next.
Sometimes a kitten will have trouble getting the hang of latching onto his mother's nipple. Maybe he keeps falling off, or he gets jostled off a lot by his siblings. If you notice this, you can try to help if the mother will let you. Manually hold the kitten at a nipple long enough for him to figure it out. Sometimes every nipple won't have milk, so find one that does.
If a particular kitten is consistently not gaining well, it's sometimes necessary to supplement with formula. Go to a pet store and buy kitten milk replacement, and bottles specially made for newborn kittens. A popular brand is KMR. It's important to only feed them formula that's intended for newborn kittens. As others have said, do not give kittens or cats cow's milk, egg yolk, or human infant formula, or anything else that's not especially made for kittens.
If you must do this, don't put the kitten on her back to give her the bottle. This can cause serious problems. Copy the natural position she'd use to nurse from her mother: on her stomach at a slight upward angle to the bottle. Make sure to always keep milk covering the part of the bottle near the nipple, so she won't drink air.
It's perfectly natural for kittens to wrestle and fight with each other while nursing. Do not worry about this at all unless one kitten is consistently being pushed away and isn't gaining weight.
It's also normal for kittens to twitch. It looks strange, but this is their nervous system's way of learning how their legs work.
Another thing you might find interesting is that newborn kittens can't go to the bathroom by themselves while they're tiny. That's why you see their mother licking their bottoms. If they were orphans, you'd have to do that yourself every few hours, so thank goodness mom is there!
(Of course it will be best if you can have your mother cat spayed after the kittens are weaned. Ask around - there might be a low cost spay-neuter clinic, or even make phone calls to see if any vet will do this at a reduced rate. (I do understand that it might be unaffordable - this is a very widespread problem.)