Since someone asked, here’s what I can remember. Walking time probably 2.5 hours. Your money back if you die of hunger or are eaten by wolves.
Head to Roquetes more or less via Via Julia metro (couple of excellent bars in the pedestrian street leading up to it), walk to the top of the shantytown and then up over open ground (there are a couple of paths and staircases) to the junction of Carretera Alta de Roquetes and Carrer Lliçà (tower of Torre Baró and ugly mirador to your right).
If you want to take a detour to the high part of the shanties of Torre Baró with a couple of interesting bars and curious folk with the occasional pig and chicken, walk about 80m down Lliçà and there’s a break in the crash barrier with a path leading through the pine forest.
Otherwise to continue Cerdanyola-wards take the stone path that bisects Lliçà and Alta de Roquetes heading uphill. I think you then take the 2nd path on the left, which takes you in about 10 minutes up to the summit with splendid views over Barcelona and the Maresma. You then follow the ridge for about 5 minutes, passing on the right an old iron mineshaft on the right and a path back to an old research station on the summit. [Walk tour that includes this bit.]
In a clearing with a couple of Italian stone pines a path to the right (I think it’s signposted Can Cuyas or perhaps Sta Maria de Moncada) takes you down from the ridge. You’ll pass the cemetery on your left and then I think you arrive at a bridge over the motorway with the industrial zone of Can Cuyas on your right.
Don’t cross the bridge unless you want to go via Sta Maria, but drop down into the valley on your left with allotments and a compound containing the flock used to graze that end of the forest. Walking parallel to the motorway along a winding dirt track you get to the cemetery access road. Turn left on that and then I seem to recall a path (lots of paths) through the woods to get to Cerdanyola.
Here’s an approximation to the first part: