So we are going to start with the runes, the thing is that bard is not a very common champion, so you can find lots of different rune pages for him, but I'll tell you the one that I like most, and I think is the best for the champion. Here it is:
Ok so let's start, I think the marks is the most variable part, in my opinion you should take the magic penetration because bard has a lot of burst in the early game, but he can also constantly damage the enemy, this way you will be able to deal a lot more damage in the trades, because the marksman won't have magic resist and you will practically deal true damage to him. Then we have the seals, health is the best option, you can choose the bonus armor but I don't recommend it, bard is a very squishy champ at the beginning of the game, so this will help you to tank a bit more in the 2v2. In the glyphs you can take 3 ability power glyphs and then magic resist, or you can take full MR, it just matters if you can go aggressive or not, but you really need that magic resist because most of the supports deal magic damage and it will help you a lot. In the last positions we have the armor quintessences, that's the most important part because otherwise you will be burned into ashes by the other marksman, nothing more to say about that.
So now we go with the masteries, there is not much to say about this, but I will explain you why this is the only good option:
Thunderlord's Decree is the best option without doubts for a simple reason, it makes a really good sinergy with your meeps, that apply 2 marks of this mastery, and it will help you a lot with the poke and also the burst. I never choose the Windspeaker's Blessing because it makes you lose a lot of presence in lane, it makes you play really passive, and you don't look for a massive healer when you pick bard. You will not upgrade your healing anyways so you will not have real benefits. With that explained we can pass to the ability order.
This one is the best for me:
In this case you can really do whatever you want, obviously the first one will be the cosmic bind, but for the second you can choose the caretaker's shrine or the magic journey, I personally like this one beacause you can roam better and the difference of the speed inside of the magic journey will be bigger between your team and the enemy.
And we will end with the builds, the build that you choose will not be always the same, you will have to make changes depending on the situation, but in general I think this one is good and I'll tell you the changes I would make depending on the circumstances.
You will always buy the ability power support item, and the Eye of the Watchers is the best upgrade, you won't really need the ghosts of the other option, because you have a lot of capacity to roam and to catch people with your ult and your Q, but you can still buy it and then buy the sightstone.
I strongly recomend the Boots of Swiftness, beacause they give you a bonus of movility to roam, but you don't condemn the teamfight movility with the movility boots, so I think this is a good option.
This item is pretty good, the Locket of the Iron Solari makes you useful in team fights or in a 2v2 with a nice shield, it's nearly a core item if there is a lot of magic damage in the enemy team, but you can still buy it if there's not too much.
The Zeke's Harbinger gives you ability power, amor, cooldown reduction, and a nice buff for your ad carry, so it fits perfectly with the champ, it's specially good if you marksman uses a lot of critical srike chance.
The Randuin's Omen will make you tanky, and you will build full tank at this point now that you have those utility items, It will make you resist more the critical damage and also peel for your ad carry with the slow.
This is the build that I recommend, you can change the last 2 items for any other tanky item that you like or that fits better with the situation, it will work too.
So this is the post, I know it's really long, but I had a lot of things and advices to tell you, I hope you liked it and if this helped someone it's enough for me, if thats the case I would appreciate a comment in the comment box down below. Best of luck in your magic journey, bye!