In St Malo go from the Britanny Ferries berth anti-clockwise round the harbour to Quaie de Dinan just by the walled city. Take the little ferry, 10 min journey, 6 euro with bike to Dinard. Find rue de la Gare. There is no Gare now but cycle/footpath along the former railway track to Dinan. The site of the former railway station is now a building site so it might take you a few minutes to find your way round it. Follow the route towards Dinard.
When near St Samson sur Rance you leave the railway. There is a signed route via the village of Taden to the canal side path. Follow the canal d’ Ille et Rance path through Dinan and all the way to Rennes.
As an alternative to taking the small ferry, you can leave St Malo on the D201 to the south and follow this route to the canal cycleway start at Taden. (Visit Map my Ride site for full map and to download GPX File.)
If you want an alternative to the cycle track, using D roads where possible, Derek and Garry Smith did this route in 2004 and posted it into the CTC Routes site. Looks good.
St. Malo to Rennes, 48 miles. Pleasant, undulating countryside on quiet roads (apart from the N137). N137 south to Chateauneuf. Then follow D337 south parallel to N137 (signed cycle-route to Rennes) via Miniac-Morvan and St Pierre de Plesguen. Here cross bridge over N137, and left to Plesder then C10 south, cross bridge over N137 and follow cycle-route signs on minor roads south via Pleugueneuc, St Domineuc, Tinteniac, and Hede. Again cross bridge over N137 and continue south on D637 via Vignoc, la Meziere, Montgermont and into Rennes. Good hotels and eating places in the attractive centre of this university town.
With tanks to members of the Cycle Chat Forums for their ideas and contributions: