The preterite is one of the two simple past tenses in Spanish. Simple doesn't mean easy -- it means it is a one-word form. The preterite is often a stumbling block for students because of the irregular verbs. They often confuse morphological features of the present with those of the preterite.
It is important to ellicit some metacognitive reasoning so they are aware of what they are doing and see some communicative purpose. So, the first step is to begin with examples of the past tense in English. Use short examples of statements in the present and some in the past and ask them to raise their hand if the sentence is in the past. For instance, John goes to the store on Saturdays vs John went to the store on Saturday. For some classes, depending on age and intellectual maturity, it may be necessary to actually do a side-by-side comparison, but it can make the class more lively if such sentences are random -- and it forces them to listen. For younger students, some sort of reward system is good. Around Halloween, you can toss a wrapped candy to them as if they were a sea lion...
The next step is to focus on the present-past contrast from a morphological point of view. Write speak/spoke and talk/talked on the board and ask them which is present and which is past -- and how they know. After a few examples, they will be ready to begin.
The next step is to remind them that while English changes the whole form for some verbs (spoke), for others, it adds -ed (talked), in Spanish, the verbs change endings to show who the subject is or subjects are -- and to show when an action happened.
Then tell them that you are going to unpack the preterite in three lessons. Each lesson will show that there are three sets of endings possible, divided as follows:
1. -AR Verbs that are regular in the Preterite
2. -ER & -IR Verbs that are regular in the Preterite
3. Verbs that are Irregular in the Preterite: New-Stem Verbs.
Point out that all the irregular verbs (except ser and ir) have their own set of endings in common and that they could be -ar, -er or -ir verbs. For this reason, they have to remember (i.e., they need to memorize and you need to quiz it specifically) whether a verb is -ar, -er or -ir. You can give them ten verbs, conjugated in the present and ask them to say or write the correspodning infinitive. Poder and poner get mixed up a great deal, in my expereince).