I suggest starting with the hacker school soylent recipe [1], but eliminate the flax seed in favour of using xanthan gum and inulin as fibre sources, and make sure you blend the lecithin granules into a fine powder before making up the mix. That gives a smooth, pleasant texture and makes it very easy to drink. I'll post the full recipe on http://www.makesoylent.com/ this evening.
Yeah flax seed is a nice omega3 source as well as a fibre source. But it tastes disgustingly bitter, and even the mostly finely powdered forms are like sand in your mouth. So for textural and taste reasons, I switched to alternatives.
1: http://www.cookingfor20.com/2013/06/18/hacker-school-soylent...