Easy. Or used to be....
Exit infield under bridge at Tetre Rouge. Take first right, which is a farm road, running parallel to track. The road ends in a farm yard, so jump the fence before you get to farm and head out across the feilds towards the track, which you can see and hear in the distance. The fields are a bit boggy and you have to get across two water filled ditches, then scale an embankment and get through a big hedge - but it's worth it! You are right on the guardrail 15 feet away from cars blasting by at 180mph. You end up about 400 yards north of 1st chicane and cars have just selected top gear at this point. You'll know you are in the right place, due to a large collection of old Guinness cans littering the area.
Go under cover of darkness and be wary of track marshalls and coppers, who have no sense of humour about this kind of activity.