from: https://www.cincyjungle.com/2018/9/1/17808772/nfl-practice-squad-2018-eligibility-pay-rules-changing-teams-what-to-know
Start Date
Before teams can build their 10-man practice squad, the final roster cuts to 53 players must be done by Saturday, September 1, 4:00 pm ET. Teams can begin signing players to their practice squads on Sunday, September 2, starting at 1:00 p.m. ET.
Most teams will have their practice squads completed and announced by Monday, September 3, which ironically is Labor Day. As soon as roster cuts are made, teams will begin negotiating with the agents of the guys they want on their practice squad, but nothing is final before Sunday at 1:00 p.m.
Eligibility
The standard eligibility for a player to be added to the practice squad is he is a free agent and has not accrued two years of NFL service. A player can only be on a practice squad for three seasons total in his career (six weeks on a practice squad in a season counts as a season for these purposes).
The league does allow for exceptions to the less than two-year rule, which was expanded with the 10-player rule. Teams may keep up to four players on their practice squad who have accrued two seasons of NFL experience, which allows a fringe player the chance to stay with the team, even if he already has been in the league, while also allowing the teams to keep more veteran players.
Accrued seasons are earned by being on the active (53-man) roster for a team for six games in a single regular season.