To achieve flat dreadlocks you’ll want to create long/wide rectangular sections. You’ll want to strive for a ratio of at least 1:2 or even 1:3 for the length vs. width of your sections. The dreads will naturally form much flatter with these ratios.
If you already have established dreadlocks you can combine sections to achieve a long shape and as the hair grows out it will form flatter dreadlocks.
Do keep in mind that hair type also determines the likelihood of flat dreadlocks.