I was completely stuck, until I read this post by Ispita_Navatar

Here is how I handled it, I'm going from Opportunity & Opportunity Lines to custom Order and Order Lines BTW

First trigger on Opportunity:

trigger createOrder on Opportunity (after update) {

    for (Opportunity opp: Trigger.new){
    
        Order__c newOrder = new Order__c();
        
        newOrder.Account__c = opp.AccountId;
        newOrder.Opportunity__c = opp.Id;
        newOrder.Total_Amount__c = opp.Amount;
        
        //Opportunity lines = [];
        
        insert newOrder;
    
    }

}

The second triiger is on the custom Order object

trigger createOrderDetails on Order__c (after insert) {

Order__c ord=trigger.new[0];

// Fetch Opp Id
Opportunity oppId=[select id from Opportunity where id=:ord.Opportunity__c];

// Fetch Opp Lines 
List<OpportunityLineItem> opplines=[select id, quantity, PriceBookEntry.Product2Id 
from OpportunityLineItem where OpportunityId=:oppId.id];

    // Loop Opportunity Lines
    for(OpportunityLineItem oppline:opplines){
        
        // Create Order Line Object
        Order_Line__c OrderLine = new Order_Line__C();
        OrderLine.Order__c = ord.Id;
        OrderLine.Product__c = oppline.PriceBookEntry.Product2Id;
        OrderLine.Quantity__c = oppline.Quantity;
        insert OrderLine;
    }

}

Obviously this doesn't have any exception handling and every save of opportunity creates an order. But it should move you in the right direction.

My questions for Ispita are

1. 

Order__c ord=trigger.new[0];

Why did you specify [0] element at the end?

2. I have my head round the trigger syntax but struggling with moving the code out to a class for better reusability. I guess it's just a question of playing and reading as much as possible. Any advice?

3. What's the best way to call my first trigger from a button e.g. Create Order

Really enjoying figuring things out. The platform is very impressive.

Mike