Collect them, as soon as they fall. There is no need to climb a tree, as tempting as it sounds.
A good acorn is fresh, rather big, feels plump and heavy, the cap comes off easily and it shows no signs of damage like holes (insects), cracks or mould.
Left alone outside, the acorn will germinate in spring or, as I have observed, some rather eager quercus robur specimens will push out their taproot during late autumn already. Most sources suggest stratification ("leaving them outside" is the easiest method), but apparently a cold and wet autumn will be enough already.
- If you want to sow them in directly the garden, burying them ca. 1 inch deep in a well-mulched bed is your best option in my experience: I have pulled dozens and dozens of "volunteers" over the years... If you want to transplant them, note that oaks have a strong taproot, so dig deeply and carefully. I suggest transplanting the seedlings in early spring, right after germination. Plant enough, in case squirrels, jays or other animals step by for their share - or consider netting.
- In pots, the same rules apply: plant as soon as possible, leave the pots outside and enjoy your seedlings come spring. Don't forget to water during winter if necessary. Note that narrow, but high pots are better suited for oaks than shallow and wide ones, the kind used for roses is excellent.
There is no need to fuss with the seeds themselves, like breaking or cutting the outer shell. Another question about red oak discusses the questionable necessity in detail and I simply wouldn't bother.
Side note, speaking from experience with quercus robur:
They come in two varieties, one will drop all leaves in a very short time in autumn, the other will hold onto most of them until spring, when the new leaves literally "push off" the old brown ones. Apparently this is a hereditary trait with regional prevalence of either kind. If you like, you can observe your potential "parents" and pick whichever you prefer.