Summarize the following:
Press the A and B (Xbox One) or X and O (PlayStation 4) buttons at the same time to do so. A text box will appear. You may instead need to press RT, RB, LT, and LB (or R1, R2, L1, and L2 on a PS4) all together. Type in testingcheats true into the textbox, then press the confirmation button. This confirms your decision to enable cheats. Find the Sim for whom you want to raise at least one need bar. Hover over the Sim to select them, then press A and B (Xbox One) or X and O (PS4) to open the cheat menu. You should see several options appear. It's in the menu. This will increase all of the selected Sim's needs to 100 percent. If you want to set every Sim's needs to maximum, you can do so by using your Sims' world's mailbox:  Go to the mailbox and hover over it. Press A and B or X and O  Select Alter Needs  Select Fill Needs (world) Your Sims will still consume their needs at a regular rate, meaning that you'll need to re-enter your chosen cheat every once in a while in order to ensure that your Sims' needs are still full.

Summary:
Open the cheat console. Enable cheats. Select OK when prompted. Go to a Sim. Open the cheat menu. Select Cheat Need. Select Make Happy. Fill your whole world's needs. Remember to refill your Sims' needs periodically.