Describe how to do it: Tobirama takes his stance; hands by his sides, feet normal, standing in his pimp daddy, blue fur collard jacket. As he is standing there with all that swag, Tobirama starts kneading Katon chakra inside him. As he gathers this chakra, Tobirama feels energetic, hard to control, and gittery, most likely due to the chakra as this is not his normal personality. As he gathers the Katon chakra in his chest, his chest starts expanding at a huge rate. When Tobirama feels like he gathered enough Katon chakra, he quits gathering it as he makes a ring with his pointer finger and thumb. As he does this, he starts blowing out Katon chakra, while using chakra and shape manipulation to create a condensed stream of Katon.
How many hand seals does it have: None, unless you would count making the ring with your thumb and pointer finger.
Advantages: Very basic, simple attack. Not hard to use, many combos are used with this technique, and I personally like linear attacks or stream like attacks. No hand seals, very quick.
Disadvantages: Almost too basic, as it is very easy to counter. No devastating damage, not really a huge area of effect. Almost any Suiton drowns it, and easily counterable.
Combos and possible uses: This can be used in conjunction with Gamabunta's Oil attacks, creating a huge flame-oil combination big enough to sear a summon the size of Manda. Could be used too hide a Raiton, since Raiton passes through Katon.
How to defend against it: Basically almost any Suiton. Make your opponent lose their balance while they are gathering chakra. Throw a smoke bomb and ruin their vision, while creating Bunshin's.
Image of the technique: This is a game only technique I believe, and I cannot find any images. :|
Sorry Minato-san!