As in single-lane two-way or two-lane two-way (single-way per lane)?

Either one is pretty easy. Single-lane you just put chain signals on both sides on entering the segment. This may be all you need, but for longer stretches you'll want a sidetrack. This is just a parallel segment that's one-way on both sides (in Factorio, real life can have either direction sidetrack). This breaks your rail into segments, so a train can move up to the sidetrack and wait for a train coming the other way clear it, instead of waiting all the way back at the station for a clear path.

Two-lane is harder to build but easier to manage. Any large-scale network will probably use this, at least for busy tracks. You just need to make sure your signals are only on the right-hand side of the rail (if you want left-handed travel then put them on the left instead). Chain signal when entering an intersection and rail signal when existing, unless there's not enough room for a train ahead before the next intersection.

I've got a mix of both of these. Most of my rails to resource patches are single-lane, because they don't get much traffic. My factory is two-lane.