Can I do the mantra in my mind?
You can recite it in your head. But it is much less effective. If you want to vibrate the mantra, but for privacy reasons it is difficult, try whispering the vibrations. It is a middle way.
Is it necessary to get into trance while doing the mantra or can I even do it while walking or standing
The longer you stay in trance the better. The fact is that a spell has to be taken seriously. You have to sit down, give yourself time every day to perform the session, calm your mind.
Inhale through your nose, slowly vibrate AAAAUUUMMM while exhaling through your mouth. Inhale again and vibrate the second word, then the third, etc. I suggest no more than 36 repetitions of the whole mantra to start with.
Do this for 40 days or for several sets of 40 days. When you are finished with the vibrations, state a phrase such as "I attract to me much money all mine in a positive way for me." Say it 10 times. Try to visualize you with money in your hand, all the money you wanted to come to you, and that you finally have it, you are happy and content, you feel it, you touch it, you smell it tastes just like money.
A then like a mental movie but very very realistically taken. This visualization is spinning of pure intense white golden light/energy. The same energy of the mantra.
Can I ask you if you know how to coordinate astrologically? If you don't know, keeping it anonymous, can you write me what is the fusorarium in your area? Then I will also pass you some links that you might need for later.

If you don't know your fusorarium, more or less figure out the country you are writing from. Keep it anonymous. Don't indicate specific cities.