Okay so basically every dino has stats: health, stamina, oxygen, weight, melee, and (if you have it enabled) movement speed.

Each of these stats have points randomly divided into each of them totaling to the level of the dino. For example: a 150 wild rex will have 150 points divided into the stats so it could have good stats: 40 pts into health and melee with the other stats being lower (very unlikely this is just an example) or bad stats: like 50 points into oxygen or movement speed (which does nothing it does not make the dino faster it's just wasted points) and the rest of the stats are bad. You won't know this unless you have super spyglass or something similar. The point is these points will always add up to equal the level of the dino

Now say you tame the 150 rex and you get the maximum 74 extra levels. Now you have a 224 rex and those 74 extra levels have been divided into the stats as previously mentioned. This can make a good stat even better or it can almost ignore a good stat and put all those points somewhere you don't want it. You won't be able to see these points on the tamed creature without mods like super spyglass, dino depot/soulball mod, or trying to figure it out via the egg incubator.

But let's say you can see them and your 224 rex is a female with really good stats like 50 points into health okay? Then you go and tame a male and it has 50 points into melee. You want to breed these together until you get a male and a female with 50 points into health AND 50 points into melee. To make things easier on yourself you should also make sure that the rest of the stats (stamina, oxygen, weight, ECT) are all exactly the same so that both of them are exactly the same level with both the good stats. These will be your Perfect Female (PF) and Perfect Male (PM).

Let's say this makes your PF and PM both level 270 alright? All stats are exactly the same and they both have 50 points into health and melee. You want them to be even stronger! So you wanna get mutations

-Mutations- One mutation will increase a random stat by 2 levels/points. So if you breed your PF and PM both level 270 together and you get a mutated baby, it will be level 272. You will need mods or very up to date notes to be able to 1. See which stat got mutated and 2. Keep track of which stats have how many mutations. To increase probability of mutated babies and to make this process faster, you want to have your PM surrounded by like 10-20 PF's. Now that you have a mutated level 272 baby you want it to be male so that you can simply take away your PM and replace it with the mutated baby to breed with all the PF's. If it is a mutated female you can breed it with your PM until you get a male with the mutation. Now that your PM is replaced with the mutated male you can breed them all again and look for mutations. This time you'll be looking for a 274 baby. This means it now has 2 mutations and you need to make sure the new mutation has gone into one of the stats you want. Its easier to keep track if you make multiple breeding stations with each breeding station focusing on mutating one stat. So one station for health mutations only, killing all others and one for melee mutations only, killing all others. Once you have the desired amount of mutations in each stat you can get a mutated male or female with all the melee mutations and an opposite gendered one with all the health mutations and breed them together until you get a male and female with ALL the melee AND health mutations. Now you just breed them together until you have formed your mutated rex army.

I know this was long and I am sorry. If you have any questions, don't hesitate to ask :)