I have a 18ah battery I am going to use to run my fishfinder. My question is, where/how do I ground the fishfinder? I don't much about electric but couldn't find anything else online. I'm going to do one of those DIY battery boxes

You don't need a ground.  You want to put an inline fuse on the positive line.

Perfect. Thanks! I was already doing the fuse but wasn't sure about the ground.

Does it matter what kind of fuse I use? ATM Mini's, ATC?

Most sonars are powered by the boat's starting battery which is connected to the motor - which serves as the boat's ground to the water.  Don't know what kind of boat you have or how it's wired, whether there's also a trolling motor, etc.  but if you have interference problems coming from the engine or the TM after trying the direct to stand-alone battery, check YouTube for possible fixes.

18 amp/hr is a small battery that the OP could be using for a kayak. Most sonar units recommend the fuse size, about 3 amp with inline water proof holder is common .

Tom

I have a small 14'er with an outboard and bow mount. I got it all hooked up. Took me a while to fix a decent waterproof box for a battery of that size.  Thanks everyone