Cookers are usually high-acid varieties, so ciders made from them tend to be very tart - way too tart for my taste - unless you manage fermentation to leave some residual sugar, or backsweeten. Blend in some low-acid sweet fruit if you can. Also, store the apples somewhere cool for a while, they will lose some sharpness and get sweeter. I store my high-acid fruit until it is overripe, this intensifies the fruitiness.