Is there a technique to refilling the coolant system to avoid getting airlocks?
not really.
fill up via the expansion tank, let it settle, if it needs topping do so.
start car, keep eye on the level and top up as required.
leave car running till it gets to normal temperature.
put heating on hot so that the whole system gets run through. you may find the level drops at this point depending how good a drain was done.
then it should all be ok.
but take some spare with you as there may be some settling.
levels should be correct when the coolant is cold. it will expand slightly when hot so will be slightly high. dont worry.
some on here moan about removing the jubilee clip to drain the system. note: (on the leon) there is a drainage pipe at the bottom of the radiator on the near side. this has a knob that you just turn to drain and turn back to stop. easy.
also read the coolant container as you can get pre mixed coolant or coolant that required diluting.