Seated calf raises are an exercise to develop the lower section of the calf muscle, namely the soleus, and require the usage of a seated calf raise machine, or they may be performed using a barbell placed and held across the end of the femur near the knees.
Whichever way you decide to do this exercise, its execution is the same.
Sitting on a bench or machine, place your toes on the machines foot platform, or some kind of block. Ensure that you are holding a barbell towards the end of your femur, or if you are using the machine put your knees underneath the cross bar. Lower your heels as far as you can, keeping your toes on the platform/block. Now rise up on your toes as high as you can giving your calves a good squeeze at the top.
Of course, if you are using a barbell to perform the seated calf raise exercise then having some form of padding placed across your legs where the barbell sits is a good idea or the exercise may be uncomfortable. Also, if using a barbell the block you place your toes upon must be stable and high enough to prevent your heel from touching the floor during the downward movement.
You should also be sure that you do not lean backwards or rock during the movement giving leverage to the lift, as this will take away from the work of the calf muscles.