This is one of the most common bases in origami and it will show up in a ton of models. It’s sometimes called a square base and sometimes it’s called a preliminary base.
1) Start with the white side up and fold in half with a valley fold.
2) This is what the paper looks like folded in half.
3) Fold in half again with a valley fold along the red line.
4) This is what the paper looks like after the fold.
5) Lift up one of the halfs and get ready to push it down like in a squash fold.
6) …starting to squash it down.
7) …pushing it down completely flat.
8) This is what it looks like after you’ve squashed it down.
9) Flip the paper over so it looks like this.
10) You want to fold the big flap over to the left so there’s a valley fold along the black line. That crease will have been folded both directions.
11) You want to squash this flap down like you did on the other side.
12) …squashing the paper down.
13) The completed square base.
If you open the base up you’ll see the follwing mountain (black) and valley (red) folds.