Rub the old paste off the CPU and heatsink using a soft cloth. Be careful with the CPU - don't touch the pins. The paste should remove quite easily.
The syringe type pastes (PC World) come with a template and plastic card. Position the template on heatsink, squeeze the paste onto the edge of template card, use the plastic spreader supplied to evenly fill the cutout in the template, remove the template - heatsink is ready for installing.