O.K.,first tutorial, so bear with me peps
first off find some sutible wire(i use the sort for kettles, lights ect,)like this
strip the casing off and select two or three strands and twist/turn them together to make the main frame
then get one strand and loop around four to six times tightly and continue,
the smaller the gap inbetween loops the better for smaller scales
(28mm) ,hold the main wire and the single strand together between thumb and forefinger to get started..it should look some thing like this?
cut the loops off close to the main wire but with enough left to protrude
after that ,wrap it around a pencil and there's your barbed wire ,i normally put it over a flame(not too long) to get the shine off and make it look black/rusted
for small scale just wrap arond a thin paintbrush handle or even a straw........it's really a question of trial and error to get what you want right
this is the only example i can find to show you(54mm)
hope this helps you for future scenery