Is your friend named Phanan? Is this a roundabout way of asking for his email?
Flow? WTF?: The Well‐Adjusted's Guide To Playlists
Going on the assumption that your friend doesn't care in the slightest what is on this list and that you have free rein to do literally anything you want with it, the first thing you're going to want to do is decide how long you want this list to be. I approach single and double discs differently; the former is less likely to cover a lot of ground, while the latter gives me the license to do whatthefuckever. Since this is for a friend, I'm assuming it's going to be a single disc. For a list of this kind, I do recommend a certain level of planning. Choosing tracks for it is something I don't really want to delve into because part of the fun of putting together these lists is finding ways to make something eclectic work in spite of itself, but don't make your experience an unpleasant one. Part of this is working out a perceivable unifying theme. Themes become impractical with double discs, but for single discs, having a specific sound or purpose in mind is advisable. It could be a certain time period, genre, or lyrical theme ‐ all of the above will make the job of sequencing it a lot easier.
Once the songs you want are in front of you (I recommend having at least 10 more than you are allotted to give yourself room to experiment; classic lists hardly ever coalesce out of the first 20 songs you pick), pick out the songs you want as your bookends. Openers could be fucking epic, or you could choose to have an intro leading into an epic song. Either way, for a single disc, you want to make your time count. Keep that in mind throughout. Your closer should be equally epic, if not more so, and unify the preceding songs. Having your bookends in place will give you wiggle room to radiate outward a few songs on each side of the list before you inevitably hit that wall somewhere in the middle where the list appears to lose its purpose. In that slot, you want to punctuate the list with something attention‐grabbing that reaffirms the fact that they should be listening to what you've made. Consider it the moment when you dream it all up again before the dash to the finish. The last handful of songs should gradually intensify and push the emotional arc of your list to near its peak. If your list is an upbeat one, this may not apply. On a lot of lists, however, this is when the darker shades are thrown in. Either way, you don't want your closer to feel tacked on.
There are a bunch of tips I could give you about sequencing that I have picked up along the way, but the fact is that we all hear things differently, and even segues are subjective. There are certain methods I have found that work well for me, however. One is to listen to the individual instruments of the two songs you're attempting to link together to see if anything will carry over naturally. Occasionally, a piece of one track (most often drums) will sound similar in another and that will cause the listener to perceive the songs as a natural pair. Another, the 10 second sequencing method, is among the most common and, in my opinion, overused. Using this, you listen to only the first and last 10 seconds of a track in order to prevent tracks from clashing. By themselves, these methods can work well for clever segues, but I actually do not recommend relying heavily on them, especially on double discs where the momentum is key and people stop paying attention to the individual segues somewhere in the middle. The definition of "flow" tends to give priority to mood and coherence anyway, so remember that before you try to get clever and unwittingly tack together Isaac Hayes and Talking Heads because the bass lines of both tracks work together well in your head.
I've made so many of these lists that it has become a science of sorts, but never let it become a ritual. Playlists serve many different functions, and, humorously enough, there are even some occasions when shitty sequencing can be preferable; if you're making a list of artists that your friend has never heard of, why would you want a list that glides right by, emphasizing the list itself over the individual tracks? The response would more likely be "yeah, I dug it" instead of "holy fuck, I loved those songs by so and so." So yeah, don't let your ego/nerdy fun get in the way of the listener's experience. I probably should have written that in the first paragraph, but oh wellz.
Hope this helps.