By adding the following endings:

Yo -ías
Tú -ías
Él, ella, Ud. -ía
Nosotros -íamos
Vosotros -íais
Ellos, ustedes -ían

Verbs that are irregular in the future tense have the same irregular stems in the conditional:

Let´s take a look at the following (venir/ to come)

Yo vendr-ía
Tú vendr-ías
Él, ella. Ud. vendr-ía
Nosotros vendr-íamos
Vosotros vendr-íais
Ellos, ellas, Uds. vendr-ían

Here´s a list of other irregular verbs with their respective stem:

Tener (to have): tendr
Decir (to say): dir-
Haber (to be): habr-
Hacer (to do): har-
Poder (can): podr-
Poner (to put): pondr-
Querer (to want): querr-
Saber (to know): sabr-
Salir (to get out): saldr-
Venir ( to come): vendr-

Vivir

Yo viviría
Tú vivirías
Él, ella, Ud. viviría
Nosotros viviríamos
Vosotros viviríais
Ellos, ellas, Uds. vivirían

To live

I would live
You would live
He, she, formal you would live
We would live
You would live
They, they, you would live

Tener

Yo tendría
Tú tendrías
Él, ella, Ud. tendría
Nosotros tendríamos
Vosotros tendríais
Ellos, ellas, Uds. tendrían.

To have

I would have
You would have
He, she, formal you would have
We would have
You would have
They, formal you would have.

To form the imperfect subjunctive, take the Uds./ Ellos/ Ellas form of the preterite and replace the ending -ron with the imperfect subjunctive endings.

Let´s take the verb tener (to have) and the Uds./ Ellos/ Ellas form of the preterite:

Tuvier-on

Finally, let´s remove the ending -ron with the imperfect subjunctive endings:

Yo tuviera/ I would have
Tú tuvieras( You would have
Él, ellas, Ud. tuvieran/ He, she, Formal you would have
Nosotros tuviéramos/ We would have
Vosotros tuvieráis/ You would have
Ellos, ellas, Uds. tuvieran/ They, formal you would have

Yo -ra
Tú -ras
Nosotros -ramos
Ustedes -ran
Ellos rais
Uds. Ellos, ellas -ran

Let´s take the verb comer (to eat)

Yo comiera (I would eat)
Tú comieras (You would eat)
Él, ella, Ud. comiera (He, she, formal you would eat)
Nosotros comiéramos (We would eat)
Vosotros comierais (You would eat)
Ellos, ellas, Uds. comieran (They, formal you would eat)

You use the subjunctive to express feelings about situations, to express uncertainty, disbelief, or doubt. It can also be used to convince someone to do something.

Como si means “as if”. You use the imperfect subjunctive after como si.

Yo me sentía como si volara por los aires./ I felt as if I were flying through the air.

Adoptar

Yo adoptara
Tú adoptara
Él, ella, Ud. adoptara
Nosotros adoptáramos
Vosotros adoptarais
Ellos, Ellas, Uds. adoptaran

Hacer

Yo hiciera
Tú hicieras
Él, ella, Ud. hiciera
Nosotros hiciéramos
Vosotros hicierais
Ellos, Ellas, Uds. hicieran

See the explanation