Remove the camshaft sprocket. It is important, therefore, to make a note of the correct installed depth of the seal before removing it from the machine; using a drill, make two holes on the opposite sides of the oil seal. Use a self-tapping screw and turn it into the hole then apply the plier to the screw to remove the seal. The other way is to use the small screwdriver by twisting it to remove the oil seal while being cautious not to scratch the camshaft's surface. It is recommended to clean the seal housing and also to address any fouled areas that may have led to seal destruction such as burrs with the help of polishing. Massage the lips of the new seal with clean engine oil and then insert the new seal over the end of the shaft, force it into the housing until it sits at an equal depth with the worn one. Especially when it is hard to install the curtain, one can use a tubular drift like socket to force the seal to fit in place without damaging the lips of the seal, more so, to establish that all the lips face inwards. If the surface of the shaft is badly scored, the new seal should be pressed slightly less into its housing so that its lip goes on the unworn portion of the shaft. Last is the camshaft sprocket and 
Timing Belt installation and proper positioning.