The tying of this nymph is done using coloured nylon, and is just a matter of tying hitches. Start is simply an overhand/granny knot, ensure it is pulled TIGHT. It is best to work with the hook pointing away from you, as picture. (This nymph is not meant to represent a specific nymph, but is purely a tying guide.)
In this fly we are going to use two colours
Start the hitches with the GREEN nylon, ensuring that the BLACK nylon is inside the hitches against the hook shank, then keeping the green nylon away, add hitches with the black nylon.
Fig 4 shows black nylon threaded through loops so it lays against hook shank.
As per picture, green nylon goes outside black nylon after black hitches.
This picture shows more clearly how it SHOULD look.
Attach tying thread and wind to nylon.
Pull forward nylon, and tie down, trim nylon to make tapered thorax.
Tie in thorax cover, in this case pheasant tail.
Add thorax material, in this case dyed ostrich herl, wind to head.
Trim surplus material, pull forward p/tail thorax cover, tie down.