Hi Felipe , congratulations on your new to you Oceanus . I have done shaft alignment a few times and I'm about to do it again as I need to replace a motor mount .
A bottle jack underneath the engine is necessary to get the new mounts in , make sure it is not going to punch through anything .
Once the new mounts are in pull the shaft flange up to the trans. flange get those two inline and measure the gaps in-between the two flanges all the way around , use a feeler gauge . Of course moving the engine around to get the flanges to mate up perfect .
There are a number of YouTube videos on the subject .
Good luck , keep us posted . Mark .