First, im just going to shift your post to somewhere more relevant, have a read through from the top as it contains some answers I was going to give you.
Next, I will prefix all.my answers with
I DO NOT HAVE A BIOMAGNET 
So I am not the best person to answer this, but I’ll make a start with what I know.
well, that makes it a little tricky.
The how will.depend on what you choose.
The what will depend on the purpose you want it for
Your options are:
Listed in the thread your post now lives in (read above)
Fingertips are often preferred BUT not reccomended.
The size can be misleading
Read through the thread I moved your post from
https://forum.dangerousthings.com/t/show-off-your-magnets-titan-xg3-v1-v2-gallery/10401/184
What implant and where?
It will also depend on what magnet you choose.
There is plenty of real-estate to share the smmae hand with a magnet and RFID implant
As above
Not really even worth worrying about, it will have little to no effect
Here is a great reference (well, i very much trust it to be, I havent read it myself, but I have recommended it many times, I really should get it and read it)
@Az_F would be a subject matter expert, he wrote an ebook all about magnets, has many magnets himself, he has made accessories for them, he has also done some very interesting experiments with them and most recently sells them (abyss walker as mentioned in above post from @Aoxhwjfoavdlhsvfpzha)
Ebook thread with link to buy
Anyway, I hope that helps, at least until an actual biomagnet implantee can chime in.