Nothing about spey casting is rigid, making it key that one approaches casting in an easy and relaxed manner. Wade shallow and cast with the knees comfortably bent, in an athletic stance with the right foot forward (if you’re a right-handed caster). Casting with the knees bent facilitates better movement of the hips, to smoothly and efficiently generate maximum rod load during the sweep. If the cast feels smooth and easy, you know it’s going to be a good one.
Don’t think that wading deep is the answer to getting a little extra distance on the cast. A deep wade decreases the vertical distance from the rod tip (while in the firing position) to your anchor, the place where the line grips the water at the bottom of the D loop. The few feet you may gain by wading out further will often be lost in the cast, as the deep wade reduces the height of the D loop and negatively impacts its shape and size. In most cases, wading to knee deep is the ideal compromise between getting out in the river and maximizing casting performance.