My boyfriend has many guitars, but until recently, no straps.
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
Decide how many eyelets you want to use and mark this. I used 3 purely as it looked right! I didn't have another strap to compare with, so if you have one, make use of that reference!
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