-
Not to come across as rude, but you should mention who supplied the kit you used again ;)
I've used the woolies kits (https://www.woolies-trim.co.uk/category/117/leather-renovation) on older leather, and used fine wire wool to clean the leather. Used a paint pad to paint the leather. It is very labour intensive.
lynx-
I used products from Furniture clinic
https://www.furnitureclinic.co.uk/?gclid=Cj0KCQjw-4SLBhCVARIsACrhWLXgAMxds8264V3ptPGs28XAsx9cpJokJUsQ5Tc0bdVD7V4pWajs-4waAjTVEALw_wcBThey also do a steering wheel kit which I used on my 318i steering wheel with good results, I would use proper leather cleaning products, any oils will affect the ability for colouring.
https://www.furnitureclinic.co.uk/catalogsearch/result/?q=+steering+wheel+kitBefore
After
Fixedwheelnut
@Dammit pretty much, it is labour intensive repairing and building up in layers, eight hours for a passenger seat and 1.5 hours a rear side bolster.
1/ wash seats and dry
2/ clean with leather cleaning solvent
3/ fill cracks or damage with leather filler paste and dry gently with heat gun, rub down with mild scotch brite pad, repeat 2/ & 3/ until happy with finish.
4/ apply layer of leather colour with sponge pad or spray gun
5/ dry and rub down and repeat until happy with finish {don't rub down finish top coat
6/ there are some colour fixer treatments to protect it.