Over and over again, I say to be consistent. So much of my advice includes the "be consistent" admonition. Consistency is likely to require some sacrifice on your part and the part of family members.
This idea of sacrifice is much easier when the child is the only child. When you are sleep training with your only child, you can more easily put things on hold for a couple of weeks than when you have an older sibling itching to get out of the house and go to the park. If you have older children at home, sacrifice is still necessary, but your efforts might take a little longer. You are likely going to need to compromise and have all children make some sacrifices. New baby will have to take some naps away from home. Older child will need to play at home sometimes.
So when you are working on something new and want to get it solidified, I recommend you maintain consistency. To do this, I often pick a time I know I can easily devote a couple of weeks to the process. Stay home and letting him get it. Plan on two weeks of really committing your time to it. If there are really bad days and you have to get out, go ahead, but try to make those very seldom and definitely not more than one nap a day (if sleep training). The effort will help him get on track. I did some major sacrificing when Kaitlyn was a baby, and it really paid off.
I purposefully planned Kaitlyn's birth to be at a time of year I knew we could commit to staying home and letting her establish her schedule. I realize not everyone has that luxury, but it is something we were able to do. We were all over the place when Brayden started sleep training. It was the middle of July and family activities were everywhere, and everyone wanted a piece of our new little baby. After about a month I realized what we were doing to him and we decided to tell people "no" and put Brayden's need for consistency above our desire to please those around us. We dedicated a good two weeks to staying home and letting him get it. He very quickly improved.
Remember, you have to establish a normal to return to. When you stretch a rubber band and then let go, it returns to where it is anchored. But what if there is no anchor? That rubber band can fly to any, random spot in the room. Create a solid schedule for your baby to hold fast to. It is well worth the effort and sacrifice it takes.
The same idea applies to whatever you are working on, though the length of time you need to devote before baby is solid will vary based on baby's personality and experience with schedules. For example, if you have a flexible 8 month old learning independent playtime who already has a predictable schedule, it might only take a week to get independent play incorporated. Or if your baby is not so flexible, it might take several weeks.
While you are introducing things that you have some control over the time of day they happen, put them at a time of day you know you can most easily maintain consistency. I have independent play in the mornings. I know I will be home in the mornings. I have it at a time of day I know I am home 98% of the time. That way, it can easily remain a consistent part of our day.
It can be emotionally taxing to devote time to establishing consistency. I would just remind you that it is but for a season. It is well worth the effort to get something solidified. Then your baby can be more flexible, giving your more freedom in the long run. Stay home and work on those new discipline tactics. Then when you take your toddler to the store, you can correct him with confidence, knowing what works and what doesn't. You can attend public affairs without fear. The sacrifice gives you greater freedoms in the future. As a parent, you will often need to make sacrifices for your children. Hang in there, do what you can to maintain your sanity, and remember those sacrifices will pay off in the end.
All of that said, I also caution against locking yourself up until your child enters kindergarten. While you do want to establish consistency, remember that consistency will help you and your family to enjoy life more fully. You have the consistency so that you can go to that social function and enjoy yourself. So you can go to the park and be able to easily control your toddler's interaction and reaction with other children. You don't want to sit in the house all day every day in fear of messing up your schedule. You can have days where things are off. You can easily go back when you know where you came from.