Daily transactions
The default deadline to make all transactions (trades, waiver claims, and free agent pick-ups) for the following day is 11:59:59 p.m. PT each night. All completed transactions will then be reflected in your lineup for the following day.
So, if you add a player to your roster on Wednesday morning at 6:00 a.m. PT, he will be available to your team on Thursday.
Custom League commissioners may choose to keep this deadline or arrange their league so all transactions can only be made on a weekly basis.
Weekly transactions
If you choose to configure your league to use the weekly option, all transactions made during the week will be reflected in your lineup on that day of the week.
In order for a transaction to take effect in a given week, it must be completed by 11:59:59 p.m. PT on the day before your league's deadline.
So, if you choose to have all lineup changes occur on Monday, you will need to make all transactions by 11:59:59 p.m. PT each Sunday.
When you wish to make moves, simply click on the "Lineup" link from your team page. Then, advance through the dates to reach the next date during which you can make your roster changes (the deadline guidelines are outlined above).
Then, as you add or drop players from your lineup, you will see the changes reflected in Monday's lineup. Similarly, any trades completed during the week will not become effective until Monday nor will any intra-roster moves (i.e., switching players between active and bench).
For all leagues that do not use weekly transactions, all intra-roster moves (switching players between active and bench spots) can be made until five (5) minutes before the start of each player's game. None of the players involved in a move can have played or be within five minutes of their game's start time. So, for example, all players who play at 4:30 p.m. PT can be moved until 4:25 p.m. PT on game day.
Also, bear in mind that your league may include a trading deadline. Click on the League Settings link from your league home page to view the configuration of your league. Trade deadlines do not impact the ability to add players via waivers or free agency.