what do you mean 'manifesting'? are we talking physically seeing your energy as you're casting? [because that's not what manifesting means] i suggest reading a few pinned threads in this forum section to help you understand 'the basics' of magick. that should give greater detail to your question if you were talking about spell work. [say energy work]
if you mean manifesting as in 'i cast a money spell i found 10$ therefore the spell manifested results', it's a little tricky to help you as there's a large number of possibilities as to how a spell's end results will look. i'd say before casting a spell make a list of possible outcomes following along with money spell, you could find money, receive a check in the mail, win the lottery, get a job, given extra chores, given a raise or bonus, someone could die and leave you money, you could be approved for a credit card, you could get a special offer form your bank. these are all possible options. [as scary as some might sound] so if you find options you do not want, or worry it could come at a great cost/risk, you should consider being more specific in your casting. [example 'harm to none' many say this in the hopes the money won't come from inheritance, theft, or loss to another] also, while focusing energy into the spell, visualize your desired outcome [you working hard, getting a paycheck, parents handing you extra cash, finding money in your jacket...] so the energy has a more direct goal instead of the vague 'money now' request. after you cast the spell, be observant, instead of focused on 'i'm going to win the lottery' because while you visualized that win, the energy/universe/Gods might take the easy route and get you a big fat bonus at work. this doesn't mean it's guaranteed to happen, but it does mean your hard work will be noticed and rewarded.
To manifest is to bring something into being, or at least a more tangible form than conceptual.
As has been said previously, someone can call a synchronistic event as the manifestation of a magical work. For example, maintaining calm in a stressful situation can be seen as the manifestation of working on coping skills. Finding money after doing a money spell, as stated before, is the manifest result. The effect of a summoned entity can be called its manifestation -- its effect on the physical plane.
"thinking about something before it happens, when working on the spell."
I think what you are describing is visualization or conceptualization. This is what helps create your statement of intent, the focus and outcome you hope to achieve.
A spell, of course, is but one means of many to achieve a magical result. It is not, in itself, what magic is. Your desired outcome is known before you begin whichever method of work you choose to do in attempt to bring about its manifestation, be it a spell, a prescribed ritual in a more strict tradition, a sigil, a prayer to a deity or an ancestor, or what have you.
The purpose of the work, I have come to opine, is the act of searing the desired end result onto the subconscious, so that it is a goal and a hope not necessarily in the forefront of the mind, but one which may be worked towards without even realizing, and a result which will more easily be noticed. It is also to send the intent and the 'energy' about that intent into the universe, or to whichever deities, spirits, whomever or whatever, you believe may put things into motion to happen for you.
Had you not done the money spell (hypothetically speaking), would you have been paying any attention to the sidewalk and found the cash on the pavement? There's no way to know for sure. But the idea of having an increase in finances has been placed into your subconscious, and sent out to the universe in whichever way you have chosen.
In the creation of the spell, you visualize what it is you wish to happen. You conceptualize the result you desire. During the spell, you focus on this intent; this vision; this desire for manifestation.
I have nothing else to add. Just, I'd personally advise you if you want to start casting spells don't pick those from this website.... I'd suggest to make your own. Even in meditation, I realized that about a Chakra meditation for example, that I'm planning to work on; a personal invdnted meditation would help me more.