A Single Member LLC is considered a disregarded entity by the IRS. Therefore, you will be reporting this income on your Form 1040, Schedule C. On the Schedule C, you will report your business name in Box D and the EIN (Employer Identification Number) in Box D. This is how the IRS ties it all together. (IRS link:
https://www.irs.gov/businesses/small-businesses-self-employed/single-member-limited-liability-compan...)
While you have a W-2 job with withholding, you still may end up being underpaid and owe a penalty at the end of the year. (https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/getting-underpayment-penalty-ge...). You may want to look at this Turbo Tax link to assist in determining the need to pay estimated taxes (https://turbotax.intuit.com/tax-tips/small-business-taxes/estimated-taxes-how-to-determine-what-to-p...).
Also don't forget about state taxes if you state has an income tax.
