Replacing the cutless bearings is only part of the job, if you are going to do it right.
Get the boat out of the water.
You will need to remove the prop, since it's off take it to a prop shop and have it tuned. You will probably need a prop puller to get it off the shaft.
Then you need to disconnect the shaft from the transmission and slide it out of the boat. IF the underwater picture is of your boat, you have a single shaft and a single strut. Check the current coupler. IF you keep it, the flexible coupler will attach to that, adds about 1 3/4" to the shaft length. I had to replace the fixed couplers as well.
Have you or would you consider dripless shaft seals. Now is the time to make the change. Have you or would you consider a flexible coupler. Now would be the time to do it.
Once the shaft is out, take it to a machine shop and have it checked for straightness. You also want to have it checked for pock marks and other blemishes which could show that the shaft needs to be replaced.
NOW, you can get down to the cutless bearing. I'm presuming you are replacing it due to a lot of play in the shaft? That is really the only reason to replace it unless you end up replacing the shaft. They are usually pressed out/in. BE VERY CAREFUL of heating the struts, you do not want to deform them. Some clown, before I fired him had cut mine out and scored the bottom of the strut. Not bad enough to cause a problem, I was lucky.
Our boat has 2 struts a side and if I remember correctly the bearings are based on the shaft size. IF necessary, since you will have the shaft(s) at a machine shop, someone from the shop can go to the boat and mich the inside strut size and compare it to the outside bearing size. IF it needs to be turned down, they can do it.
We ended up replacing 2 shafts and 4 cutlass bearings and having the props tuned as well as fixed couplers. We also have dripless seals so the seals were replaced and added flexible couplers (the shaft needs to be addressed for this, so the prop doesn't hit the rudder). The machine shop made the shafts and turned down, where necessary, the bearings. The also made me a press for sliding the bearings in.
When you get everything done, and BTW, there are pictures, on line, of a shaft snapped, looks like a laser cut, and if that were to happen, you are looking at some potentially serious repairs; it all goes back in reverse of the way it came out.
Doesn't take any special talent, just some tools and some work. Tools are online or maybe the yard has them and will lend or rent them.
Good luck and enjoy the rabbit hole.
Let me know if you have any questions.