I used Debbie Bliss Baby Cashmerino with nos. 11 (UK) needles.
Begin by casting on 30 stitches, knit approx 6 - 7" (2 pieces) or 12 - 14" if knitting in one piece.
Stitch seams leaving a gap at the top, stuff, sew up, embroider edges if wanted, add button - that's more or less it! Measurements are not really crucial at all.
This needlecushion measures approx. 6.5"x 5.5" (x 2 pieces or 1 long piece).
The front is Garter and Stocking Stitch, and the back is just plain Stocking Stitch.
I knitted a heart motif into the centre - but you could knit in anything of course, and I added a large decorative button. (Click on photo to see larger image.)
You could also knit it all in stocking stitch and embroider initials on the front too.
I stitched the seams, leaving a gap at the top and stuffed it with wadding and also embroidered a line of chain stitch on each of the long sides to make the edges look neater.