How would I start the process to filing back taxes?

[VolvoGirl]

What years?  

Online is only good for the current year.   You can buy the last 3 years of Turbo Tax here,

https://turbotax.intuit.com/personal-taxes/past-years-products/

You will need a full Windows or Mac to install it on.

How to prepare a prior year return

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-prepare-a-prior-year-tax-return/00/25...

You will have to print and mail in prior year returns.  And if you are doing more than 1 year, send each year in a separate envelope. Sometimes the IRS will just staple all the sheets together and miss a second return.  Be sure to attach copies of your W2s and any 1099s that have withholding on them.  Don't forget state.  You have to mail federal and state in separate envelopes because they go to different places.  Get a tracking number from the post office when you mail them for proof of filing.

Or if you want to file for free and have a simple return you can get the blank forms from the IRS and fill them in by hand.  I have a list of some of the basic ones I can post direct links to.

[VolvoGirl]

You should start with the oldest year first and then continue on to the next year.  You only have 3 years to file for a refund or you won't get it.  No limit if you have a tax due.  As I said above Turbo Tax only sells the last 3 years so if you need to file 2019 or earlier you need to go to a local tax place or do it yourself by hand.