Whatever you do, please don't try to skid with those straps.
I had a similar pair that I tried when I first tried out some straps on a fixed.
I gave it a few test runs on a test hill and my foot popped off and pulled the velcro off.

Luckily, I practiced emergency stopping (A soft bail that was left foot in frame and right foot in rear tire) before I went more than 10mph fixed,and I was on a test hill (easy decline, and almost no cars).

Real, actually useful straps are expensive but they are absolutely necessary.
They usually go for $50+ but I have found used ones for $20 on ebay. You just have to be patient.
Crawl through some forum posts for good suggestions.

The ones you bought will be ok for general foot retention and to see if you like the feel of straps or not, but for something long term you're going to want something else.