Now —

When you're giving instructions to someone, you can say "Now —" at the beginning of a new step in the instructions. For example, if you're teaching someone how to bake cheesecake:

Now — mix all that up for a minute or two, until it's nice and thick.

You usually only use this in spoken English. In writing, you start a new step with "Next," or "Then..."

paddle (one's) arms

To "paddle" in the water means to hold your fingers together to make a cup shape, and move your hands back and forth. This is the way that most children are first taught to swim. This style of swimming is called "the doggy paddle".

A paddle is a stick with a wider flat part at the end, like what you would use in a small boat. This is where the verb "to paddle" comes from.

kick (one's) legs

You can "kick (something)", like kicking a ball. But in the water, you can also "kick your legs". This means to move them back and forward quickly.