You could tape the materials both sides of the joint to prevent it spreading if you can't get it accurate enough.
Use less than you think you need - i.e. cut the nozzle thinner than you'd think, the mess comes from excess, though obviously ensure you've filled the joint. Use a smooth continuous movement with the gun when applying to avoid lumps or excess in certain parts.
As mentioned use a wet finger to seal the joint, dipping finger in washing up liquid helps avoid stickiness, and you can get a little plastic tool to help but I find them useless.