Editing always requires the longest time of any step in the process. There are very few photographs that I know will make the series as soon as I see them, most will stay there indecisively, shuffling around, getting cut out and brought back. Shuffling the order is important, it forces me to see new sequences and connections where before I didn't. Some new photographs make their way in the selection like this.
If I feel I'm getting stuck, I'll leave the selection for some time, to let it sit while I work on something else. This affords me more perspective and a bit more of an objective eye.
The hardest photographs to edit are the ones I'm not quite sure what to do with, they're not bad enough to exclude but something about them isn't good enough to keep. That's why I'll also involve someone around this time to help me with edits. Whether they're a photographer or not is irrelevant but it's always someone who knows my photography and whose judgement I trust. Too soon and the series isn't coherent enough yet or there's too much material to expect anyone else to reasonably sit through.
The idea is to arrive at the most interesting set through successive iterations of selections.
Some hard decisions involve excluding strong pictures that are perhaps too strong for the set (they stand out too much and overshadow the rest). I also tend to leave some pictures in the set which I'm more emotionally attached to that might otherwise not really belong there. For some pictures, the only way to really tell whether to exclude them or not is by making a print. At various points in this stage I'll go to the darkroom with a 10-15 item list of frames to print.
Printing helps me further edit down a set. It really puts on paper what I had in mind when I first shot something, or reinterprets it in a new way. Either way, the print is the reference for what it will look like in the set. It almost never looks like a copy of the negative on paper, or even of what I had in mind when I shot it. It's more like a transformation and that is what helps me decide to exclude or keep some photographs.
If it seems like editing is mostly discarding photographs, it's essentially that for most of these stages.
Arranging the set in a sequence to me is about coherence and balance. Coherence shows in the choice of subject, type of scenes and printing (I don't tend to mix high contrast and softer prints in the same set for example). Balance means I don't include a lot of photographs that seem too close or repetitive, that I don't have too many vertical or horizontal shots, that there's variety in alternating between wider shots and closer ones, that there isn't a dominant tone between prints (not too many dark or bright prints).
Most importantly, I try to balance the really strong photographs, especially those that tend to stand on their own with others that work better in twos or threes. The strong, individual elements tend to be a bit slower in terms of the rhythm of the series while those that come in pairs flow easier. I have to keep in mind how these two elements affect rhythm when constructing a series.
I don't shoot in a narrative way so an exact order is rarely important. I do however sequence prints together or farther apart to highlight tension or bring together two or three frames that complement or contrast with each other. Those then form blocks of elements that can be moved around in the series.
Finally, the opening and closing elements are also very important. I try to always open with a strong picture, to lead into the series. The closing picture tends to be softer, like an open ended question.