How to knit a square on the diagonal...

[karenarp]

How to knit a square on the diagonal...
without it turning into a diamond.

Please help!

I have tried making several different squares starting in a corner and increasing one stitch a row thereafter using different yarns and different needles yet only a few of them are coming out 'square' while the rest are a mild diamond. With some shaping and pulling I can get it to look more square.
What I am wondering is if this is normal when knitting squares?
If it is not normal can you guide me to how to fix it as I make it?
Do I need to only increase one every second row or increase two rows, do a third row no increase then repeat?
I am certain it is not my needle size.
or is it just a tension thing that will fix itself over time, as I practice more?
I have included a photo of my first 'diamond' square. Should be square.

[Ewezer]

After getting it to the required number of increased stitches are you then doing the other half by decreasing at the start of every row until you are back down to the number of stitches you started with.