How do you fix a laguna 1.9 dci head gasket?

If you don't know how to do it in the first place - then don't bother trying. Fixing a head gasket is far too complex for an amateur - go round some of your local garages and get a quote for fixing it. It could end up costing thousands of pounds/dollars!

My advice is - if it turns out to be pricey, then just buy yourself a new car if possible - and not a French one! Something like a Toyota Avensis (which I have had 3 of - all of which got near 200,000 miles) or a Honda Accord are good value for money, and VERY reliable (the V-Tec petrol engines in the Accord are probably the most reliable engines known to man).