I've done most of it myself for 20+ years.

The best piece of advice I got from a mentor who was also a 1099 guy was this:

At every deposit from a client into my business checking account I take a red pen and subtract 25%-30% and highlight the entry. Yes, I keep paper records - including a check register. I use this lower amount as my account balance for available funds - knowing I've got my taxes "paid".

Every quarter at estimated tax time I go back check off those debits - add them back to my account balance - then write the estimated tax checks. Never get hit with an "unexpected" tax bill.

Very old school - but it works.