The thing you gotta know about Jarman or as he is known as by his friends 'J-swizzy dizzy da big I Vizzy' is that he is only to be played by people who don't have sandpaper up their vaj. This means no *****ing at your team and always going in with your e-q if you see an opponent.

You may be lookin at the items and thinking 'Hold up G Swaggy you stupid ****, what kind of build is that?' but after trying and testing EVERY BUILD IN THE GAME ON J-SWIZZY I HAVE CONCLUDED THIS ONE IS THE BEST. Basically you wanna pick up the banner of command early so you can have to flags in your inventory, your own one and the banner. They have amazing synergy and you will dishin' out flags wherever you go, this is the one part of the build that is in no way flexible, it must always be built first and immediately. Do not buy any starting items as it will slow down your path to the banner. Next up you wanna get either a frozen mallet, a frozen heart or a seraphs. Just remember, you have to get the frozen items in a row, so if you get one you have to get the other straight after, not going for the seraphs or you may miss out on that ice cold freezin boost. Basically, the seraphs is to synergize with your W. It costs a **** ton of mana, so seraphs clearly helps with that, and the shield your W gives is smaller than teemos ****, so seraph also makes up for that. People often complained J-Swizzy Dizzy's W was his weakspot, the only thing it did well was slow, and that's the only thing seraph's does badly, so they compliment eachother nicely.
The frozen items are to get that extra CC. Unfortunately, CC is an area where J-Swazzle falls, he only has his knock up, slows, and ring of rock as cc, so the slows from these two items will seriously secure you a ton of kills. Unfortunately, you spend a lot of gold on useless tank stats on these items, but overall I think the slows make it worth it.
The swifties help J-McFizzyDizzy gain that extra juice in his boots, which compliment his slows nicely. Always buy boots 5th, you don't need them early game.
The last item is another banner. This will really shock your opponents as they will not be able to handle 3 flags and they may spam the chat saying 'wtf 3 flags?' to which you can reply with a giant slap in their face with your heimer-****er.

Please use victorious J-****er for this build, as it gives you that extra victorious boost. GLHF!