One great exercise to get started on Double stops is by Sevcik called Preparatory Exercises in Double Stopping Op.9 for the Violin.
In this tutorial I go through etude no.3 which is exceptional for pinpointing intonation in advance, correcting when combining the interval and for finding the correct bow arm angle.
Work on the intervals slowly while concentrating on intonation. Memorize the position of your right arm when on the double-stops and make as little movement as possible when moving over to the intervals. Apply even pressure on both strings when playing both notes together.