Hello! I worked as a W2 music teacher, a contract music teacher and a musician. I use my new guitar for all three. Where is the best place to write off my entire guitar?

[Opus 17]

No where.

You must allocate your expenses between the 3 jobs, and any expenses allocated to the W-2 jobs can be deducted.

If audited, you need some reasonable way of allocating the expense and proving it.  (The IRS does not have to give any deduction that can't be proved.)  Number of hours worked at each job, perhaps, or percentage of income earned by each job.

[scamposanto]

I hardly use my guitar at my W2 job as they have guitars at the school to use if need be. Can I just allocate it between my contract teaching job and musician job?

[Opus 17]

If audited, you have to be able to prove your allocation to the auditor.  There is no deduction allowed for personal expenses, and the deduction for W-2 expenses was eliminated by tax reform.  Whatever you claim now is at your own risk.  Most people aren't audited, but you should keep copies of your records, including your argument for how you allocated expenses, for 6 years.