..."you would need to be very smart in all subjects."
Pshaw. Nope!
Outsourcing. There are MANY different ways to do this. From outside classes (there are classes in every subject at every level taught by pros out there),to family members/friends, to co-ops, to tutors, to interactive learning programs (like mathUsee or time4learning). Homeschooling doesn't USUALLY mean you teach your children everything. It more commonly means that you're the PRIMARY teacher, but that you ALSO act as a conductor.
Although... there's a line from the Bitter Homeschooler's Wishlist that is pretty apropos:
#12 If my kid's only six and you ask me with a straight face how I can possibly teach him what he'd learn in school, please understand that you're calling me an idiot. Don't act shocked if I decide to respond in kind.
http://www.secular-homeschooling.com/001/bitter_homeschoo...
Because, carts and horses here... as long as you know how to add, subtract, multifply, and divide... you've just covered the ENTIRE math curriculum for the first 3-4 years of public school. Worry about algebra and geometry and trig and calc AFTER they know how to add and subtract AND you've figured out what you enjoy learning and what you prefer to farm out to someone else (dad, grandparent, friend, outside class, co-op, tutor, interactive online programs, etc.)
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Supplementary programs? Oh. Gosh. GAZILLIONS (like toys in a toystore)
For math alone I have maybe 40 or 50 (all from k-prealgebra) that span from pbskids and other websites that are totally free, to ClueFinders / Carmen SanDiego (and other CD games, at about $5-10 each), to complete montessori albums (free) from prek-alegebra including how-tos and pictures on how to make your own manipulatives, to partial curriculums (like Singapore Math, MathUSee, TimeForLearning), to workbooks of many many different kinds. Sooooooo much out there.
BIGGEST DON'T IN STARTING HOMESCHOOLING (that most of us fail miserably at our first year)
DON'T GO BUY A BUNCH OF STUFF.
Learning styles and teaching styles aren't sorted yet. Spending a bunch of money out the gate is usually all "lost" money. Free trials. If something doesn't have a free trial (where the kids can use it, and you can see how it works)... think 2 or 9 times before buying it. School-in-a-box curriculums are the "worst" at hundreds or even thousands of dollars. I mean, a $10 computer game... no biggie. But $100, $250, $700+ dollars for untested materials? Don't do it. Even if it's a "deal" for the whole year, instead of the monthly cost. THAT AND THERE IS SOOOOO much out there that's free and BETTER than the expensive stuff.