When you make boxes, one of the first things you will learn from experience is that any piece not perfectly square is going to make the rest of the project very difficult to complete. The next thing you will learn is to make opposite sides at the same time, top and bottom, left and right, front and back and label each on the outside face to keep track of them and their desired orientation. A light pencil line label or piece of blue tape with the writing on it works very well. Pencil lines can be removed with alcohol, if you don't press so hard that the pencil leaves a dented line in the wood.
If the opposite sides are made the same and at the same time, a slight error in squareness or dimension isn't so critical as long as the two pieces are kept oriented the same way as you assemble the box. Orient one side so it's angled edge is opposite of it's mate and nothing is going to go together without a fight, and the battle scars will be visible after assembly. Just like a roof on a house, both ends of the roof must be sloped the same. You can't build the house with one end of the roof sloping the opposite direction. A small slope won't be as visible as long as both ends slope the same way.
You will also learn after building a few boxes, that it's best to make the box bottom and box top together, and then cut the top free after the box is glued and assembled. After assembly and gluing, I always draw a light diagonal line from box top to box bottom on the face side of the box before I cut the lid off and then I use my table saw to cut the top free by cutting opposite sides first, then inserting spacers to fill the saw kerf, then cutting the remaining two sides holding the bottom of the box tightly against the saw fence as each cut is made. This diagonal line will help you keep the facing side of the lid and bottom oriented the same when it comes time to install the hinges and latch. A box that is a slightly off square toward being a parallelogram will look fine if you don't put the lid back on it facing backwards.
If cutting box joints for the corners, again make the two sides at the same time, beginning the box joint cuts with a space on the bottom of both ends of each piece. Do the same with the front and back pieces, cutting both ends the same, except begin the cuts on the bottom end with a pin. Doing this will assure that the corners of the box will all go together, and a side benefit to this is that you can decide just before assembly which face of each piece will look better when faced outward and you can face the side that isn't so nice toward the inside of the box, but you can't make this choice if the two sides aren't perfectly square.
It's going to take making a few boxes before you get the technique fully figured out, so use cheap pine for your first few boxes. Set your present box pieces aside and go back to them after you have this experience. By then you will have the ability to square up the pieces of hardwood and assemble a box correctly. If you continue trying to build this box now I fear that you will only destroy some expensive wood or you will have a box that is a total eyesore.
I now make quite a few boxes every year, and I thoroughly enjoy the process, but I had to make several boxes out of scrap wood before I had my system fully figured out and could make boxes that I like quickly and easily.
Show us some photos of your completed practice boxes and we will help you avoid the mistakes for the next boxes, but please use cheap wood for your first practice boxes.
Central North Carolina