There are many ways to boost morale, but I think this will cover all of the major points and a few minor ones:
It's good to do this with the builder legacy so you can get free water and power, both of which are good for morale.
Many survivors get a morale boost from having a garden, workshop, infirmary and/or watchtower. A kitchen, lounge, and latrine are good for community morale, while outdoor beds are, for most survivors, bad. Choose your base and facilities accordingly. I used the Corner Office in Lowl in Cascade Hills. It comes with a kitchen and watchtower. I put two upgraded bedrooms indoors and built a garden, workshop, infirmary, and lounge (large slot) outdoors. I had six survivors, one of which had the cooking skill to make feasts in the kitchen. (If you uprade the cooking skill to "cuisine," that's even better.) I added an Xbox to the lounge.
Four of my six outposts were Dirty Bird espresso stands.
Some community members will provide a morale boost upon becoming promoted to hero. Failing a community members' personal goal will impose a morale penalty.
Make alliances with enclaves. Many of them will give you a morale boost perk or something that will contribute indirectly to morale.
Most of the above morale boosts are permanent or long-term. Additionally, you can boost morale in the short term by killing hordes, freaks, and plague hearts. Too many infestations will impose a morale penalty.
Destroying all plague hearts, thereby eliminating the blood plague, gives each survivor in your community at that time a permanent morale increase. New community members do not get this boost.
Once your long-term morale is in the empowered or enthusiastic range by following the above tips, it's time to load up on temporary boosts. Throw some parties in the lounge. It took me a while to realize that multiple lounge morale boosts can be going on simultaneously, and the effects do stack. Prepare a feast in the kitchen. Wipe out enough infestations to at least remove the penalty. Go out and destroy a plague heart. (You will typically kill a few freaks and hordes in the process of doing that. )
Be aware that the stacking effect tapers off as your morale approaches 100, so when you get to 98 or 99, you're going to have to add another 5 or 10 points to get it to 100.