How thick of steel you will be welding on will determine the amount of amperage you will need.  You will need your tig torch with a gas valve on it (300 amp torch) to be DC negative and your ground will be positive.  You need a nice sharp tungsten.  Not sure what 300d you're talking about and if it has remote capabilities.  If it does then a remote would be nice, but you don't need it.  You'll need a gas block that your tig torch will screw into and your gas line into the other end.  This will let you hook a stinger to the block and make the tig torch hot.  If you don't have a remote then you will have to be old fashioned and flick start you arc with a piece of filler rod.  Then should should be off and welding.;-)