In answer to your questions about the process and decisions about actually making the new space marines see the below quote. Note this is for brand new chapters created from scratch.
According to their charter, each Chapter is obliged to send 5% of its
genetic material to the Adeptus Mechanicus on Mars. This ‘tithe’ has
two purposes. Firstly, it enables the Adeptus Mechanicus to monitor
the health of each Marine Chapter. Secondly, it enables the Adeptus
Mechanicus to store gene-seed with a view to founding new Chapters.
A new Chapter cannot be founded overnight. A single suitable gene-seed
must be selected for each zygote. Zygotes are then grown in culture
and implanted into human test-slaves. These test-slaves must be
biologically compatible and free from mutation. Test-slaves spend
their entire lives bound in static experimental capsules. Although
conscious, they are completely immobile, serving as little more than
mediums within which the various zygotes can develop. From the
original slave come two progenoids, which are implanted within two
more slaves, from which come four progenoids and so on. It takes about
55 years of constant reproduction to produce 1,000 healthy sets of
organs. These must be officially sanctioned by the Master of Adeptus
Mechanicus and then by the High Lords of Terra speaking for the
Emperor. Only the Emperor can give permission for the creation of a
new Chapter.
https://www.warhammer-community.com/2016/11/16/rites-of-initiation-the-making-of-a-space-marine/
With regards to resources, home planet etc I imagine that is all decided on and implemented during that 55 year period, it might also be dependent on the reason for the chapter creation. For instance if a chapter is to sent immediately to war then I imagine their home planet is not the immediate concern.
As far as Doctrine all marines should follow the Codex Astartes as a base, they will then probably add their own slant on this based on the inherent traits that come from there geneseed, for instance Imperial Fists will naturally find themselves forming defensive lines and taking a measured approach, Whitescars successors will find they love being on fast moving bikes. This will be the case regardless of whether the chapter knows its origins or not.
Training, remember that all marines are trained through direct insertion of tactics and methods to their brains (can't remember the right description for this) so all marines will be ready set up to go to war. The chapter Master and officer corps will be selected using the normal selection process, this is a new chapter so I imagine the first set of marines will be from inductees identified as strong officer material.
In terms of oversight, all space marine chapters are watched, closely, so any new chapter would simply move into this process.
