i used a nissan thermostat replacement, and used a silicon style selant for use in cars. comes in a little bottle with a brush.
leave that to dry recommended time before starting anything with coolant etc.
1. drain radiator and fluch out with hose from top and bottom hoses.
2. take the theromstat housing out and replace thermostat, then re-gasket back in place.
3. replace thermo housing, and all hoses
4. think the system is 10L for skylines, so work out how much coolant you will need as per mix instructions on bottle. add that much coolant to radiator.
5. top up with water til its full
6. turn ignition on and set heater to full.
7. start car and let it run to heat up. topping up radiator as it drops.
8. once system is near to temp and doesn't seem to be needing anymore water, put radiator cap on
9. unplug the bleeder bolt, without removing it, til the bubbles stop coming out and only water is leaving the thread. do it up and away you go.