In my experience, I have always divided sprint planning into two sessions, the first session where I carry out the capacity planning exercise with the team considering the available capacity, leaves/holidays, and existing sprint spillovers, and the second session where the planned work for the upcoming sprint is discussed, reviewed and stacked in a prioritized order to take only what the team can accomplish.   Throughout the planning session, the underlying principle always follows is that the ones who do the work estimate the work item i.e. how the work will be done; while the business helps in a broader understanding of what the work item is and why it is important. Tools that are used during planning include agile management software, capacity planning sheets, estimation tools such as planning poker, T-shirt sizing, etc.