The method proposed by GVA is probably the fastest though a little convoluted. Indeed follow the transit signs, and just before the arrivals duty-free, go upstairs (there may or may not be a security check, probably not) to departures. At the right of the duty-free shop there's a passage, not very clearly marked, to the F gates, which are those in the French sector. You'll end up in the section leading past French customs into the French sector. Go through the security check (if you're business you can use the business line, but most of the time there are so few people I don't bother).
The alternative and probably more official route, is to go landside to the Swiss sector, up one level to departures, then walk to the northern end of the terminal and walk through the one-way doors to French customs to the French sector.