It is important to ensure that the milk and oil are chilled properly as you would do for making pie crusts.
It is equally important to handle the dough as little as possible as the warmth will cause the oil (or butter if you use it) to melt and affect the texture of the biscuit.
While rolling out the dough, do not be tempted to use more flour than the absolute minimum necessary. Adding more flour will make the biscuits heavy/ dense in texture.
It is important to use a cutter with very sharp edges, or the sides of the biscuits will get pressed down and will not rise on baking.
Place the cut out biscuits quite close together to ensure that they do not spread outwards much, but rise upwards if you want taller biscuits. Also ensure that your baking powder is active.
Usually the biscuits are brushed with an egg wash before baking, as this ensures a beautifully browned and shiny top. You may brush them with milk instead.