First of all you take the nous form of the present tense, for instance "jouons"
Now take the "ons" of the end of the verb which leaves you with "jou"
The endings of the imperfect are : I : ais We : ions You (s) : ais You (p) : iez He/she : ait They (p) : aient
Add the apropriot ending on to the stem that you created by removing the ons. Je jou - ais
Je jouais Tu jouais Il jouait Nous jouions Vous jouiez Ils jouaient
This rule works for all verbs apart from etre which is iregular.
If you follow the other rule for etre the nous form is nous "sommes" but infact what you need to use is "et" you use this as your stem.
The endings are the same so it will go... Je et-ais