Cubeshape is usually the hardest step for beginners to master.  At first, you’ll end up twisting randomly for minutes on end on some shapes.  It’s inevitable.  I did it when I started.  Practice a lot, and scramble a lot.  Watching the shapes while executing computer-generated scrambles is a great way to build familiarity with the puzzle and improve your cubeshape.
Get your square-1 in hand.  Solved if possible.  At least solve the cubeshape.  Now, do a single / move.

This shape is called kite/kite.  It’s the only shape than can go to square in one twist, so to solve any other shape you first have to get to kite/kite.  If you see this shape, you’re almost there.  Line up the layers exactly like this and twist to solve cubeshape.
Now, take that solved puzzle and do /(3,0)/.

This shape is called parallel fist/fist.  It’s one of only two shapes that can go to kite/kite in one twist (and therefore to cube in two).  You can solve it by reversing what you just did (so /-3,0/ )  You can also do any of the U/D and F/B mirrors.   Basically line up the seam between two edges and the seam between two corners and twist.
Sometimes you’ll try this, but instead of kite/kite you end up with fists again.  Sorry.  This means you have a case that I call “bad fists”.  You can tell because the U and D layers are mirror images of each other.  This is one of the worst possible cubeshape cases and luckily, because of the current scrambling method it doesn’t occur often.  You’ll have to solve it a different way, which I’ll teach you soon.
The other shape that is 2 twists from solved is called barrel/barrel.  To generate it, do /3,3/ on a solved square-1.

To solve it, just hold it like it is now, one horizontal, one vertical, and twist.  If you did it right you should end up with kite/kite.  If you can’t solve that I’ve failed you.
The next shape you should learn is called paw/paw.  Do /3,0/-5,2/6,6 on a solved puzzle to see it.

Again, solve it by AUF/ADFing to this position and twisting.  There are two seams that are each between a pair if corners, but only one is actually twistable.  Line up both layers parallel and twist to get fist/fist.
Like fist/fist, paw/paw also has a “bad” version where the layers are mirror images of each other.  And like bad fists, bad paws must be solved differently from good.
Next is scallop/scallop.  See it on your puzzle with /3,3/-1,-2/.

Nice and symmetric.  Line it up just like this on the line of symmetry and twist to get back to barrel/barrel.  You’ll notice when you get two symmetric shapes like this the best course is usually to split them right down the middle.  More on that in a bit.
Take your square-1 and do /3,3/-1,-3/

This is called muffin/muffin.  To solve it, all you have to do is line up both layers and then shift one layer by one notch.  As long as the muffins are facing the same direction but not lined up, you’re good.
One last shape for you to learn this section.  Do /3,3/-1,0/ to generate shield/shield.

It’s solved like you see it now: line up the seams between two corners but make sure the layers are not parallel.  You can also do (6,6) and line up the seams on either side of the bottom point.
These are a few of the basic shapes you should learn.  In the next section I’ll teach you a method to do any shape.  But the ones I’ve shown here are so easy you’d be wasting your time using the other method.  Plus, these shapes are part of the path you’ll need to solve harder shapes.
Cubeshape Part 2: Group Corners

So now you know how to solve the easiest cubeshape cases.  But what if you get a case that you don’t know yet?  The answer: group corners.  The goal of this substep is to get 6 corners on the bottom and one of 5 cases on top, and then fix each case with an optimal algorithm.
The way I go about this is simple: first, build a group of 3 corners and store it on the left half of the bottom slice.  Then, using only U layer and / moves, build a second group of 3 corners and insert it into the D layer.  This part is still mostly intuitive.  Then you’ll end up with one of the following cases.  These cases are named by the number of edges grouped together between the 2 corners on the U layer.

8/0:  Symmetric cases are very easy to solve in this method: just split down the middle and you’ll get a case you already know.  In this case, it’s scallop/scallop.

6/2:  Another symmetric case.  Splitting this one down the middle gets you paw/paw.  You should know how to finish this one.

4/4:  Last symmetric case.  Splitting straight down the middle gets you shield/shield.

7/1:  This case I call observatory.  To solve it, position it like this and do /2,-2/-3,-4/4,-3/-2,-1/-3,0/  You’ll notice that it goes through fist fist, and it actually hits a symmetric case called kite/scallop after 3 twists.  Don’t just learn the alg, watch what it does to the shape, so you’ll already know how to solve all the intervening cases.

5/3:  Hold this case like so, and perform /4,0/-5,-4/4,-3/-2,-1/-3,0/  Notice that it’s the same as the last alg except for the first 3 twists.  Once again, you’re getting it to kite/scallop and then solving symmetrically.

Now that you’ve returned the puzzle to cube shape (except for possibly the E slice, but don’t worry about that for now), you’re ready to head over to corner orientation.