Notes
1 Accusative Masculine - use Genitive if object is human or animal
2 Watch out for 7-letter spelling rule (ы must not follow к,г,ч,х,ж,ш,щ use и instead)
3 Some masculine nouns exceptions have plural ending а or я. For example глаз > глаза, город > города, дом > дома, голос > голоса, доктор > доктора, поезд > поезда, профессор > профессора, учитель > учителя
4 Except those ending in же,це,че,ше,ще which end in а instead
5 If ends in ж,ч,ш,щ takes ей in the Genitive e.g. нож > ножей
6 If ends in я but proceeded with a consonant in nominative singular, takes ь in Genitive e.g. неделя > недель. If ends in я but proceeded by a vowel in nominative singular, takes the ending й in the Genitive plural e.g. партия > партий. If ends in consonant cluster insert a fleeting vowel in between e.g. бабушка > бабушек, сестра > сестёр, земля > земель
7  If ends ие take ий in Genitive plural e.g. здание > зданий
8 Also takes this ending in the Genitive if ends in ц (and if stress on preceding syllable)
9 If consonant cluster, vowel is often inserted e.g. окно > окон
10 Masculine nouns ending ец lose е when declining e.g. конец > концы (and in all other cases)
11 Masculine nouns ending ец and stressed on stem have ев in Genitive since о can appear after ц only in stressed positions e.g танец > танецев
12 Masculine nouns ending ок,ек,ёк lose fleeting vowel in declension e.g. рынок > рынки (except some exceptions which don’t)
13 Some feminine plural nouns which have consonant cluster take o between the consonants in Genitive e.g. марка > марок, бутыилка > бутылок, вилка > вилок. Between clusters containing ж,ч,ш,к - e is inserted e.g. ложка > ложек
14 Feminine nouns ending ья lose ь in Genitive e.g. статья > статей, except гостья which is гостий in Genitive
15 Letter я is replaced by а in Dative/Prep/Inst. if ь appears after ж,ч,ш,щ e.g. вещь > вещам (Dative) вещах (Prep) вещами (Inst.)
16 Exceptions брат, лист, стул follow this pattern in all cases except Nom., but with ь inserted before ending e.g. братьев Nom is братья. Exceptions муж, сын, друг also follow this pattern in all cases except Nom, but with ь inserted before ending. But in Nom, has ья e.g. мужья, сыновья, друзья
17 Masculine nouns ending адин/ядин replace last two letters with е and in Genitive remove last two letters e.g англичанин > англицчане (Nom) англичан (Gen). господун also loses last two letters in Genitive.

Remember - foreign origin words don’t decline