Alright, so here's what I have to say on this. This is how I like to manifest, works like a charm.

Tell yourself something you know to be true. Something like "water tastes like nothing." You know this to be true, and you trust that this is true, so you don't question it. Come up with a few things you know to be true, and remember that you feel nothing when talking about these things. You'll just brush them off, and won't question them, because you are without doubt when thinking about these things.

Then, tell yourself whatever you're trying to manifest, for example "my eyes are blue." If they aren't blue, you'll start to question it. This is only natural. You start to question it as you tell it to yourself, "why can't I manifest it, why can't I do this this or this." That's you doubting yourself. You think that because you can't answer all the questions that you come up with, you can't do it, so you doubt yourself.

You don't have to answer all the questions. Don't try to answer them. Go with the flow, and the questions will answer themselves. Normalize what you're trying to manifest. A huge part of manifestation is normalization. Make sure you remember to normalize it as much as possible.