I never had to grind any sockets to get the head off. Some people may need to grind down a ring end spanner to remove the front cylinder head bolts (depending on brand and clearance).
Drain the coolant at the pump first. Then remove the plastic connection that joins the head. You can leave the hoses attached. The rads can be left on the bike, but will need to be loosened at the top and bottom bolts removed to allow them to move forwards.
Remove the powervalve while the cylinder is still on the engine. Much easier to pull it out. Be careful not to pull up on the actuator arm (although not so much a concern if you'll be retensioning anyway). The side to side clearance of the valve can be changed by shimming the assembly. Make sure you clean the assembly up really well, and that you leave some play as it does require clearance for carbon buildup. If its not noisy then I wouldn't change it.
Squish clearance, gasket stacks, port timing. Read up. Now is the perfect time to have the head worked.
Measure your ring end gaps of the old ones to gauge the wear on them. Set the new ones by using the Wiseco calculator. For a 300 you're looking at around 0.4mm to 0.5mm. Use the piston to push the ring down in the barrel before measuring. The cylinder can be cleaned up by using WD40 and a scotchbright pad to remove any glazing. Then wash down with detergent. Inspect for wear/damage. Ideally you'll have an engine rebuilder measure the bore to determine if its still in spec and what size piston to use. Others will have more info on this.
Consider cleaning up any casting slag from the ports. You don't want to mirror finish or knife edge anything. You can polish the exhuast port to a mirror to help reduce carbon build up if you have time on your hands. Make sure not to grind too close to the nikasil coating. It may chip off and you'll be up for more $$.
Inspect carb boot/intake manifold for any signs of perishing or cracking. Pull the reed block out and inspect petals for wear.
The bottom end can have some side to side movement but should have zero play up and down.
With gaskets a light blow with a rubber mallet can help to break covers/parts free without damaging the gaskets. Make sure you replace the head o-rings and cylinder gaskets with fresh ones. Others can probably be reused if not damaged. A light smear of grease on them when installing means they'll come off easier next time and also helps to hold them in place during installation. Trim the base gaskets if required. You don't want any excess gasket blocking off any ports.
Thats all I've got for now. Heres the thread I ran while doing my last one. Probably goes into way more detail than you need.
http://www.gasgasrider.org/forum/showthread.php?t=11970