Automate the approval process?

you can Automate the approval process by using this piece of code in your opportunity trigger:-

if(oldMap.get(oppObj.Id).Fire_Custom_Approval__c == false && oppObj.Fire_Custom_Approval__c == true){
Approval.ProcessSubmitRequest req1 = new Approval.ProcessSubmitRequest();
req1.setComments('Submitting request for approval!');
req1.setObjectId(oppObj.Id);
Approval.ProcessResult result = Approval.process(req1);
}

Hi,

To add more about Process builder -

Using Lightning Process builder you can make the record automatically submit to the approval process whenever it meets the criteria.

• Go to setup->Create->Workflow & Approval->Process builder
• Then select the object to which you want to perform the auto-approval submission
• Enter the criteria for the record
• In the set Action select 'Submit for Approval'

Hope this helps.