There are two ways to do this, you can either use "würde + infinitive" or a one-word form. The one-word form is identical to the past simple (Präteritum) for regular verbs, so it is only used for auxiliary and modal verbs. In this case, except for the verb sollen, the thematic vowel has an Umlaut. For all other verbs the würde + infinitive form is most commonly used.