Formula fields help please

Hello all,
I am need with help with my formula, I want it to do a calculation based on a criteria using formula fields this how my formula looks:
IF(
IF( ISBLANK( npsp__Primary_Contact__c ) , value_if_true, value_if_false),
IF( Payment_method__c NOT( CONTAINS(paypal, debit card, mastercard, stewardship, work for good ) ) ,
IF( StageName CONTAINS(post, secured) ,
IF( Contact__r.Funding__c , yes)
Amount - ( Amount * 0.8)
))))
Any help on how I can this working please
If this criteria is met:
- primary contact is not blank a field
- payment method (picklist field) is not paypal, debit card, mastercard, stewardship, work for good
- stage (picklist field) is either posted or secured
- funding (picklist field) equals yes
If criteria is met I would want this to be calculated Amount - ( Amount * 0.8)
Any help would be appreciated.
Many Thanks