I prefer E. Burns method of sew then trim. I've done both. I do the old measure, cut, sew for a scrap log cabin one. Too much work. I save 2 inch strips for that now.
Yes, do use a diagram. Pretty easy to find on the internet - just type in Log Cabin Diagram.
To get started, decide on what look you want. I followed a style from Jean Wells using golds and tans on the light side, and dark greens and brick reds on the other. Some do blues on one side and whites on the other. If you have a color collection, just post about it. Someone can give you a suggestion to start.