Please note: You can mate at any time you want, but mate when it says to. Otherwise you will probably die at least once.You can also stray a little from what this says, but this is the easiest way that I know to complete the stage.

First, when you emerge from the meteor, eat meat or plants to complete the eat mission.
You should grow in size.
Keep eating until a cell eats another and reveals a gold shield. Swim into it and then keep eating. You should mate to equip the part in the gold shield.
After you grow, find a Booster and try to defeat it. Boosters are red and have more than two eyes. It will unlock another gold shield. swim into it and then keep eating.
after you grow again, find a cell with a pink trail coming out of it. defeat it and stay out of the trail. It will unlock yet another gold shield. swim into it.
then, keep eating until you grow.
find a pink cell named pinky or punky and defeat it.It will unlock yet another gold shield. Swim into it.
After you grow two times, find a Buzzy. They are orange. It will unlock yet another gold shield. Swim into it. Then keep eating until you finish the cell stage. Create your first creature and scramble up onto land.