Using TurboTax Self-Employed Online, you can enter your expenses in the Income & Expenses section on top. Go to Federal if you already have your tax return screen open. If you you can't find it, click on Tax Home first. Then, click on Take me to my return. From there, click on Federal on the left side. Then, tn the top screen, click Income & Expenses. Look for the Self-employment income and expenses section. Click on Add or Edit. There, you will be able to enter your business expenses in the Expenses section by clicking on Add expensed for this work.
[see above edited answer on 02/06/2018 at 4:18 PM PST]