Ashtabula cranks are the easiest thing in the world to fix. Your average dude with his tools rumbling about in the kitchen drawer can do it with a big wrench and a screwdriver.
Loosen the nut on the left side crank near the bottom bracket. Use a screw driver to tighten the bearing cone (you can usually see the slots that look like they were made for a big screw driver. Then, tighten the nut again.
1.) You have to tighten just enough that the crank is not loose in the bottom bracket. Don't over tighten or you will make it hard to bicycle and you will ruin the bearings. Don't worry. Every wrench learns this way. Just experiment.
2) If you are feeling pretty smart, add some grease to your bearings when you have the nut off. Just turn the bearing cone counter clock-wise to loosen, the bearings will be exposed good enough. You can use bicycle grease, automotive grease, or even petrolium jelly (Vaseline). If you don't have a grease gun you can use a butter knife to wiggle some grease onto the cones.