The Silmarillion gives little detail about the northern part of the Exiles' journey, saying only that they '...passed over the mountains, and came down into the Vale of Sirion...' (Quenta Silmarillion 23, Of Tuor and the Fall of Gondolin). There were two possible routes from the northern Encircling Mountains into the Vale of Sirion, both of which were under the power of Morgoth at this time, and both therefore difficult and dangerous.
On this map we show the Exiles following the western route, down through the Pass of Sirion, which would have been by far the more direct way into the Vale, and perhaps the less perilous. At one time Sauron had watched this pass from his tower on Tol-in-Gaurhoth, but that tower had been destroyed long beforehand, and for the Exiles the route would have at least been passable, though hardly free from danger.
The alternative would have led the Exiles eastward and southward, though the dark woods of Taur-nu-Fuin and then down through the Pass of Anach and along the course of the river Mindeb to reach Sirion between Brethil and Doriath. This rather more circuitous way was known to have been used by Orcs, so it would have been no less dangerous than a journey through the Pass of Sirion. In principle it seems the rather less likely option, but it would nonetheless match the brief description give in the Silmarillion.
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