From Dunkerque port, you can follow the EV12 route as far as the Belgian border, which will get you to Adinkerke/De Panne. It's well signed for cyclists and is traffic-free for the most part (going through Dunkerque town itself has sections on shared paths beside roads, but not actually on them).
Tip: When leaving the port at Dunkerque, exit through the left hand gate then you can turn left at the roundabout. Then follow this round, past the new roundabout/road which has recently been built, and then turn right onto a short gravel road (which soon becomes tarmac again). This gets you to Loon Plage and the EV12 without having to go along the busy road with all the lorries leaving the port.
From Adinkerke you could go up to De Panne itself and then take the Kustroute through Koksijde to Nieuwpoort, or you could go inland via Veurne. From Nieuwpoort follow the R1 which will get you all the way to Brugge. Again pretty much traffic-free.
From Brugge to Ieper I'd head south towards Torhout in the first instance, then you can choose from there depending on which particular part of the Salient you might want to go through. You could carry on through Roeselare towards Menen and then go up the Menin Road, or loop round a bit and go through Passendale (past Tyne Cot) then Zonnebeke and on to Ieper, or maybe also via Langemark where the German cemetery is.
