I want to start waxing my board but never did it before.

just wondering if those plastic wax scraper with a square edge is
really useful.

can't I just use something like a putty knife?
At the hardware store, I found a 12inch long paint guide ($4) - it has
a 12inch x 4inch metal edge and plastic handle. I would think that
would scrape stuff better than a square piece of plastic.

please let me know if you've used a plastic scrapper and if it's worth
$8 - $10.

$8 - $10 seems like a lot for a plastic scraper. The most I've paid is $4
or $5.

I tried a putty knife and it flexed too much which made it skip which can
nick your base.

I find a "ski sized" scraper works better than a "Snowboard sized" scraper
for this reason as well - less flex, less skipping, wax comes off more
evenly.