In order to fish for karambwans, you need to collect some karambwanji, which is the bait of choice.
With some cash on you, input the command
;;shops in the chat. From there, you can talk to Paul if you are an ironman player, or Gerrant otherwise. Purchase a karambwan vessel (used for the next step), as well as a small fishing net if you do not have one already.
Paul's Ironman Shop
Gerrant's Fishing Shop
With your small fishing net, run to the teleport wizard located in the center of Edgeville. Under Skilling teleports, select Karamja.
You will be teleported to the east side of Karamja.
Run west until you see an open pathway to the banana trees.
Run north and you will find the fishing spot for karambwanji. They stack, so you never have to worry about banking.
The amount of karambwanji caught depends on the player's Fishing level, increasing every five levels. The exact formula is 1+floor(level/5). This means players with a Fishing level of 95 or above will catch 20 karambwanji at a time, and boosting beyond 99 will result in 21 karambwanji caught per catch.