How to form the cases ? Here I try to sum up all Latin noun case forms, without dividing the nouns into declensions. It can be a general review of Latin declension. Sadly, we cannot always foresee the form of genitive singular. The other forms are easy to foresee, with only small exceptions. Maybe this version can be helpful. Some exceptional words are excluded.

The accusative is same as nominative in all neuter forms. In masculines and feminines : -us changes into -um, -a into -am, -es most often changes into -em : locus > locum, causa > causa, res > rem. Other nouns have accusative in -em, but sometimes with change of the form of the noun : legio > legionem (same change as in genitive). More details about them below.

The genitive is given in dictionaries. The rules are: -a changes into -ae, -us and -um into -i (II declension), -us more rarely into -us (IV declension), -es into -ei (V declension), -u into -us. Most nouns in -er also have the ending -eri or -ri (II declension). With other noun endings (and also neuter nouns in -us, like tempus), it's more complicated – the ending is most often -is, but the form of the noun can change. Most frequent changes : -o > -onis (more rarely -inis), -en > -inis, -is > -is, -or > -oris, -as > -atis, -rs > -rtis, -ns > -ntis. Examples : causa > causae, locus > loci, bellum > belli, exercitus > exercitus, res > rei, cornu > cornus, legio > legionis, pars > partis, urbs > urbis, hostis > hostis, flumen > fluminis, tempus > temporis.

We can get the dative from the genitive : -ae, -ei stays, -i changes into -o, -is into -i, -us into -ui : causae, loco, bello, exercitui, rei, cornui, legioni, parti, urbi, hosti, flumini, tempori.
The ablative can be also formed on the base of the genetive : -ae changes into -a, -ei into -e, -i into -o, -us into -u. The ending -is changes into -e or -i. So : de causa, loco, bello, exercitu, cornu, re, legione, parte, urbe, flumine, tempore.
The vocative is different from the nominative only in case of nouns in -ius and -us of the II declension (nouns which have -i in the genitive) : -ius changes into -i, -us into -e.

Nominative plural : We have a special rule for neuter nouns. They get -a in this case : starting from the genitive : -i changes into -a, -is into -a or -ia, -us into -ua. Masculines and feminines : again we take the genitive: -ae, -i, -us stays, -is and -ei changes into -es. Our examples:
masculines and feminines : causae, loci, exercitus, res, legiones, partes, urbes ; neuters : bella, cornua, flumina, tempora.
Vocative plural is always identical with nominative.
Genitive plural : easiest from the ablative singular : -a > -arum, -o > -orum, -e > -erum, -u > -uum, -i > -ium, -e > -um or -ium : causarum, locorum, bellorum, rerum, exercituum, cornuum, legionum, partium, temporum, fluminum.
Dative and ablative plural : from the genitive singular : -ae > -is, -i > -is, -us and -is > -ibus, -ei > -ebus: causis, locis, bellis, rebus, exercitibus, cornibus, legionibus, partibus, temporibus, fluminibus.
Accusative plural: in neuter nouns always identical with the nominative plural. Also in cases when the nominative plural ends in -s (endings -es, -us). In other cases, we form it from the nominative plural: -ae > -as, -i > -os : causas, locos, exercitus, res, legiones, partes, urbes ; neuters : bella, cornua, flumina, tempora.