After coming from the KV-85 and IS, this line will not play like the previous vehicles at all as there are many major differences compared to other Soviet heavy tanks. The first vehicle of this line is the IS-M, which will give you some insight into how to play this and the following vehicles.
Compared to the IS-3 on the same tier, the IS-M features similarly thick armour, but a different armour layout. Instead of having a pike nose, it's only sloped vertically. The effective frontal armour value is about the same and the lower frontal plate is just minimally thinner than the upper. However, the front can be penetrated by other heavy tanks and tank destroyers rather easily, which is why you must angle your tank.
The side armour is pretty strong, which you can use to side-scrape. Even if this tactic isn't one of your personal strong points, the IS-M is rather forgiving in this aspect, making it a great choice if you're learning how to side-scrape properly. Another thing worth mentioning is the decent mobility that allows you to get to important positions fast enough.
The top gun is the 122 mm D-25BM that is slightly weaker than the IS-3's BL-9 in terms of penetration and accuracy but features a better reload and aiming time. Apart from that, it's what you would expect from a Soviet gun – at mid and long range, it won't hit much, let alone moving targets or enemy weak spots. Stick to close combat where the bad accuracy doesn't matter as much and you can make the best out of the good rate of fire.
Because of the gun being as precise as a shotgun, it's evident that the equipment needs to balance out this weakness as much as possible. The set above will make the gun more reliable and shoot more often.
Despite all the differences between the IS-M and other Soviet heavy tanks, the choice of crew skills is similar. In fact, Repair, Smooth Ride, and Snap Shot are way more important; getting tracked with a rear-mounted turret will leave you completely defenceless in many situations. Also, as the dispersion is higher than usual, the bonuses of the two latter skills are more effective as well.