Problems with guitar straps:
They cost money and are big. Vinyl parts on guitar straps are famous for removing the finish from guitars. It might fall off the little buttons on your guitar and wreck your instrument while you bicycle through traffic. The buckles might scratch your guitar in the bag. They might make clicking sounds on something right in the middle of your perfect solo take.

Here's a quick strap that has none of these flaws. You can make it out of almost anything.
It's easy to adjust and very secure.

Step 1: The Tail End of the Strap

Find a chunk of strap a meter or a bit longer.
Find a couple of pieces of string a few decimeters long. (yay decimeters!)
Shoestring would be plenty strong. I'm using thousand-kilogram dyneema here.

Fold the end of the strap over and sew it, making a loop. If that's too much trouble you can just copy the head-end knots starting in step 4, which don't involve any sewing.

Or if you've got a leather belt from a fat person or a really long belt from the 80's, just poke a hole through the end.

Step 2: Surgeons Knot

Put a piece of string through the strap loop and tie it with a surgeon's knot.
It's just like a square knot but with an extra turn on the second half-hitch.
It's less likely ton untie by itself than a square knot. This dyneema is slippery.

"Mud Lake" is the most popular name for lakes in Minnesota.
"Surgeon's knot" is probably the most common name for many different knots.
If you know this knot by another name, please comment.

Step 3: Hitch It to the Tail Peg Button

Loop the string into a clove hitch and hook it onto the tailpiece button.
"American Boy's Handy Book" calls it a "builder's hitch".

Step 4: Head End

Poke your other cord under the strings at the head end of the neck.
Tie it into a loop with a surgeon's knot just like before.
Then form it into a lark's head knot as seen here.

Maybe your guitar has a button at the other end of the body. You can tie your loop there, or tie it higher on the head by the tuners. I like to do it this way so it lies flat on my back when I'm carrying it.

Step 5: Larks Head Adjustments

Pull it snug and it's very secure.
But you can grip the knot and slide it up and down the strap very easily.

That's it!
You've got the best guitar strap money can't buy!