You can see XKCD’s visual explanation at this link... what-ifDOTxkcdDOTcom/6/
Pinch a small loop of line and slip it under the plastic straw loop, then pull it through and over both ends of the straw and draw the line tight.
Solar fire methods work best between the Spring and Fall Solstice because of the angle of the sun... the higher in the sky, the better... a great noon-time project.
Thanks for sharing candidly your urban stealth camping approach... good insights that can be applied.
Thank you hafiezismail for posting my video... I've enjoyed reading the comments. Survival situations are not only in the wilderness, so this would most likely be an urban scenario following a natural disaster. I'd be pleased to have you visit my website featuring other survival techniques. :-)
Try seeing the bigger picture of alternative ways to use your resources... in a time of emergency, fire could make a huge difference. Check out my site where my blog and web pages explore other ideas.