I prefer the term "group 2 verbs" to "-ru verbs." Group 2 verbs are verbs that end in -eru or -iru, not just -ru. Of course there are some exceptions, like kaeru, which ends in -eru but is actually a -u verb. Conjugation is easy: remove the final -ru and append -masu, -mashita, etc.

"Group 1 verbs" (again, -u verbs) are verbs that are not group 2 verbs. Conjugation is a bit more difficult because the -nu, -bu, -mu, and -u verbs have many suffixes. However, after memorizing these two (-nbmu and -u, because -nu, -bu, and -mu are almost the same), the rest are easy.

There are only two irregular verbs: kuru and suru. Just memorize them.

I learned Japanese by just memorizing. Once you have memorized enough verbs and their conjugations, you can figure out the conjugation of a new verb even if you don't understand how it works.