This diagram may help you when tying frets. The frets decrease in diameter as they move up the neck to allow for the amplitude of the vibrating string. How thick the frets should be is a question of personal taste. I like to start with a 0.9mm diameter, providing the height of the nut allows for this. Then I continue with two each of 0.8mm, 0.7mm and 0.6mm.
To ensure the fret sits tight, it is tied one position above its destination and then moved up. If tying the ends proves difficult it may help to use a pair of pliers. The knots are tied on the bass side of the neck so they will not get in the way while fingering the notes.
The ends are cut back leaving only a few millimetres which are then carefully singed with a lighter. A little mushroom shape will form and stop the knot from opening.
The knot can be practised by tying a piece of string to a handrail.