Ideal is a large manufacturer of wire nuts, their instructions are:
Installation Instructions
1.Turn off power before removing or installing connectors.
2.Strip connectors to the correct strip length per the chart
3.Straighten and align frayed strands.
4.Hold wires together with ends even. Lead stranded wires slightly ahead of solid wires.
5.Pre-twisting acceptable, but not required. For pre-twisting;
strip wires long, hold wires together with insulation even,
twist wire ends together, trim to recommended strip length.
6.Insert bundle into connector and twist clockwise until tight