This is the prototype of making a guitar strap WITHOUT having to use fixings from a pre-bought strap (which I found common among other DIY guitar straps).
I used what I could find around the house:
For the Strap:
Fabric - (I used fairly strong cotton)
1 slider buckle ~2.5" wide - (I used one from an old bag)
For the Attachment Points:
1 Doc Marten Boot - (mine were 14 hole but you really only need the top 3 from each side)
Strong cord/leather thong
2 belt sliders ~2.5" wide
You also need:
A sewing machine (or needle & thread if yr feeling industrious)
Stanley knife/strong scissors
Step 1: Make the Guitar Attachment Points
Check how wide this is - I made my strap 3" wide (with seam allowance), so cut the leather at around 2.5"
Draw a line from this point, parallel with the top of the boot mark out a rectangle measuring 4" in length.
Mark 2.5" along this and draw another line to divide the rectangle into a square (containing the eyelets) and a smaller rectangle.
Mark the halfway point on this line and use that to mark out a pointed tip
Cut this out using strong scissors or a craft knife
Make a slit just under 1" in the centre - this is where you will attach to the guitar, so start with a small hole, and make it bigger if required! I found 1" was was pretty secure
Step 2: Make the Strap
You can either use this length (+1-2" seam allowance) or multiply by 1.5 if you want to make an adjustable strap.
Mark this on your fabric along one edge
Decide on how wide you want the strap to be - I based it on the width of the buckles I was using, and also by measuring the distance covered by 3 eyelets on the boots I was using for the attachment point.
The width I used was 6" - I folded this in half along its length to give a final width of 2.5".
Fold a 1" hem along each short edge, iron, and sew with straight stitch.
Fold a 1/2" hem into each long edge and iron.
Fold the strap in half and pin the hems together so that the right side of the fabric is showing.
Sew along the pinned hem.
Check the width will work, and add a few more lines of stitching down the length of the strap for strength and to help it hold its shape, and sew along each end to securely seal it.
At this stage you should have a nice flat strap!
Now check it is the right length...
...if it is, move to the next step!
...if you want to make it adjustable, get ready with the slider buckles!
Thread one about halfway along the strap - this will be your main adjuster slider
Thread the second one about halfway long again - this you will use to attach one end of the strap to the guitar.
Take the end of the strap and fold it over the middle bar of Buckle 1 (see picture) and sew securely.
The strap should now adjust!
If you are having problems figuring this out, take a look at any other adjustable strap you may have on bags etc around the house - I may have used a particularly round-the-houses method of doing this!
Step 3: Attach the Ends to the Strap
You now have a strap... but no way of attaching it to a guitar...
Take your slider buckles and attach one to each end by folding the end through the middle bar of the buckle.
Using strong cord or leather thong through the eyelets, attach the leather ends to your strap.
p.s. you may notice from the photos that I deviated a bit from this... it was a prototype after all! I only had 2 buckles, which I'd already used making the strap adjustable, so I used an eyelet punch and some press-stud fittings to make some eyelets in the end of the strap to which I could then attach the leather end. I think It would be better if you have 2 buckles tho!