You absolutely need to know WHAT you want to manifest and WHY you want to manifest it.
The what is quite self explanatory; make sure you know exactly what it is you want to manifest and please, by all means, be specific. Don't just say 'I want to lose weight' - it's far too general. You can lose an arm and technically speaking, you've now lost weight, right? So you want to be more specific than that. Do you want to lose fat? Do you want your muscles to pop out more? Do you want to fit in to a specific dress? What exactly is the end goal here? The more specific the what is, the easier it will be for you to manifest it and also to know when it happens.
Another thing that you need to know is the WHY of what you're manifesting. Sometimes the why figures itself out - for instance if you want to manifest a bigger house because your current place doesn't have enough space for your growing family, the why is pretty obvious. But what if you don't have a big family but still want a bigger house? This is where the why can become more tricky because you actually need to tune in and figure out why it's so important to you. And the why doesn't have to be groundbreaking or awe-inspiring - it can literally be because you desire to live in a home that reflects the luxurious life you believe you deserve.
This is where you need to start practicing the arts of SURRENDER and FAITH.
We can in theory control all of these things, but (and it's a wonderful but, if you ask my husband) we're not meant to. You know why? Because God can do it so much better and with greater impact than we ever could.
It is so tempting to want to take control of this but whenever we do, whenever we try to force our timeline and ego-driven expectations onto our manifestations, we usually end up making it far more difficult for us to get it OR, if we do get it, end up making it less enjoyable and soul-led than we wanted it to be.
Many people want to manifest financial wealth. And as infuriating as it might seem for me to say this; making more money is easy and you can do it whenever you want. But creating lasting and joyful wealth, although it also can be easy and you can do it whenever you want, requires from you to surrender more to the divine desires that you have been gifted by God and have faith that the wealth will come when you get out of your own way - read: when you stop letting your expectations of how, when and where the wealth will come from limit the blessing.
For instance, we see someone who has incredible financial wealth - wealth that we desire to have as well - however, we also see the person working a job they hate or an insane amount of hours that take away their time and ability to enjoy the wealth. This is not soul-led wealth. - this is forced wealth.
On the other hand, we see someone else who also has incredible financial wealth but for them, the wealth seems to come easily and effortlessly. They don't resent the source of their wealth and it doesn't take away from their enjoyment of it in this life. This is divinely gifted and soul-led wealth.
So allow God to do what He does best: allow Him to lead and guide you in the direction that will bear the most treasures for you in this life and the hereafter. And as long as you know the what and the why, you're well on your way to align your intentions and actions for a much desired outcome.