I use my corners as guides. I work with each side of the cake as its own section, and I know each corner will have a vertical 'post', so I eye-ball each side in half, and make a small mark with my spatula or toothpick, and then in half again, and again, until my marks are about 1in apart (or whatever look you are going for). Then I know I won't have to stretch to make it fit together at the end.
Then to make my horizontal 'weaves' by starting at the bottom. I don't like to start at the top because my cakes aren't always 100% level, but I do know the bottom of my cake is. I do one across, then above that use my tip as a spacer, then another line across on top of that and so on and so on. Hope that makes sense.