Starting my first independent contractor gig and I have a complex situation this year

I retired in March of this year. I will be starting to work for the organization I retired from as an independent contractor on a part time basis the first week in September.

Prior to retiring in March I had earned approximately $18,000 in income as a W2 employee. I also started to collect social security in March, my monthly income is $3200 prior to the 10% income tax deduction I am having with held. My income from from the independent contractor gig will be $800 per week.

How much will I need to set aside for the self employment and income tax? How and when should I start to make estimated questions.