kmw199614 the connections on the IMU especially look very small and hard to get solder
You got it right. It is indeed the hardest part to solder.
I suggest to start by looking at some video tutorials on SMD soldering. You will find plenty online, and they are often full of little tricks and considerations that would be difficult to explain on text.
For example in this one the general process of SMD soldering using solder wire is nicely explained
This guy dedicates a significant amount of time to talk about the solder he is using. And many SMD soldering tutorials do it as well. That's because the type and quality of solder will have a significant impact on your soldering process. In this video he's using Lead free solder, but saying that he's not a great fan of it. Leaded solder is definitively easier to handle (but it is under strict regulation in some parts of the world)
And here's another video that shows the specific process to solder a component with a package similar to our IMU. The difference is that our LSM9DS1 with its LGA-24 package doesn't have a big center pad, so can omit all the considerations about that.
(I'm pasting the video starting from the soldering part, but I would recommend to watch it from the beginning)
For more considerations about soldering take a look at this answer.
If after seeing the videos you have an doubts on the equipment of techniques used please share them with us
Another videos will use solder paste instead of solder wire. That's another completely valid technique