As a French, I'll do this in the next weeks:
Go to the website of government where I can pay all my taxes.
Enter my credentials (I got them my snail mail years ago).
Answer a few basic questions about marital status, address, spouse, age and number of children (very quick, the Fisc (our IRS) know everything and it has not changed).
All incomes from the family (employers, stocks...) are already known and pre-filled. I just have to check that it is in sync with all the summary papers that my employer or my banks have sent me these last weeks ("you must declare XX € in field XY, and YY € in field YZ"). If I want to check, I'll have to make some additions (hard!). I don't remember many mistakes since all this is already filled.
Tax deductions have their own fields. I must sum the numbers from the papers sent from by charities. The nanny for my daughter is already subsidized, so they know how much I've paid and can deduct.
The biggest challenge was tax deduction for the heating system and some insulation in the house. The problem is knowing if and how I'm allowed to deduct these expenses, not computing them. If I think that the 10% default for professional expenses is not enough, I can count all kilometers to work and a few other things. You need Excel to track all this, not more.
At the end the website tells me how much I'll have to pay, or how much I'll get back. If I want to calculate myself, the rules are simple enough that Excel should suffice. My grand father, whose situation was much more complicated, did all of this himself without computer.
So, French administration is on this rather simple and effective. Well, 500 years ago, foreign ambassadors were stunned by the efficiency of our tax system
I see comments from Switzerland or Finland telling this is not more complicated there. I read German newspapers and every year the tax sofware is a hot topic - but these people never knew how to make things simple (not a surprise that SAP was born there but I digress) (said as Germand-friendly Frenchie).