How can I keep my dog from freaking out whenever I use my printer?

Give her a treat and reassure her whenever your printing something, make her know that it isn't something to be scared about, stroke her. Don't shout at her because that will scare her more. It could take time so be patient.

put her on the leash and put her in front of the printer. give her a treat. walk away with her still on the leash and walk back to the printer and give her a treat. repeat 5 times. during those visits to the printer let her smell and lick it.

after 5 times, turn the printer on keep her on the leash and then pet the printer and talk to it like: "oh GOOD boy" she will look to see since you arent scared she shouldn't be. whenever she is silent when the printer is on give her a treat.

maybe even put a treat on top of the printer when it is off, so she will think of it as a good thing

Try to show her its nothing. dogs will usually always have something that they will bark and not like inside the house. for my dog its the vacuum cleaner. That for him. is the scariest thing lol.

Try have her sitting next to you and give her a reward so she calms down. Or a dog toy with treats inside. keeps them entertained and busy

god bless