It's hard to answer this because there's really no such thing as a serf. What serfdom means, its status, rules and obligations, differs quite significantly depending on period and location. The idea of 'freeing' a serf is also based on quite modern ideas of what 'freedom' means and might be quite different to what a medieval peasant understood by the term. In 11th Century England, for example, a servus was a literal slave, whether an agricultural labourer or in domestic service, and whose hopes of emancipation mostly lay in the posthumous beneficence or legal censure of their owner. The pop-cultural idea of a 'serf', a tenant farmer tied to a manorial estate through obligations of service, was a villein, 'villager', smallholder, bordar, or 'cottager' depending on their relative wealth and status.

Slavery in England appears to have been abolished by the mid-12th century, but the various tiers of tenant peasantry still held different privileges and obligations from even their contemporary neighbours in France, and indeed which could differ based on their immediate and personal relationship with their tenant-in-chief compared to their neighbours'. It was common in France, for example, for villagers to use a manorial oven in the baking of their bread, for which they were required to pay a fee, while the custom seems to have never caught on in England, despite the influx of Frankish tenants-in-chief in 1066.

Modern concepts of 'serfdom' don't really necessarily reflect the nature of actual tenant relationships on the ground. Pop-culture assumptions often posit a nebulous 'serfdom' as a form of somehow being 'owned', but certainly within an English context, it was more of a reciprocal relationship in which land was leased in return for a service rent obligation. Establishing 'freedom' could be as simple as just leaving. Laws dating back as far as Alfred of Wessex's Doomboc prohibit a lord from granting any form of tenure to somebody who is already in the service of another lord without their permission, but even if you couldn't guarantee receiving that permission to leave, there was every chance that a different tenant-in-chief would grant you a tenancy and refuse to send you back. An easier way would.be to simply go to the nearest city, although you'd likely be unable to go back to your original lord's land without facing legal recourse.