public with sharing class OpportunityApprovalCtrl {
/*  The "with sharing" keyword will prevent unauthorized users from succesfully saving changes to Opp. Only your Approver
    and anybody "godlike" (System Administrators, users with "Modify All Data" permission etc) will be able to approve
    (http://salesforce.stackexchange.com/questions/5377/are).

    If you'd prefer some explicit spelling out of "you don't have permissions, the buttons will be disabled for you" - shout.
*/
    public String approvalComments {get;set;}

    // Mapping between our picklist values and the actions in actual process.
    public final Map<String, String> validActions = new Map<String, String>{
        'Approved' => 'Approve',
        'Conditionally Approved' => 'Approve',
        'Rejected' => 'Reject',
        'Recalled' => 'Remove'
    };

    private ApexPages.StandardController sc;

    public OpportunityApprovalCtrl(ApexPages.StandardController sc){
        if(sc.getId() == null){
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.FATAL, 'Please pass valid Opportunity Id in the page URL'));
        } else {
            this.sc = sc;
        }
    }

    public PageReference save(){
        try {
            progressApproval();
            return sc.save();
        } catch(Exception e){   
            ApexPages.addMessages(e);
            //  This probably would need some savepoint & rollback (so we don't stay in limbo of "approval OK, Opp update failed")
            //  but let's keep the example simple (lol).
            return null;
        }
    }

    private void progressApproval(){
        Opportunity opp = (Opportunity) sc.getRecord();

        if(!validActions.containsKey(opp.Approval_Status__c)){
            throw new ApprovalException('Unexpected action: ' + opp.Approval_Status__c); // Somebody was messing with the form?
        }
        if(opp.Approval_Status__c == 'Conditionally Approved' && String.isBlank(opp.Approval_Comments__c)){
            throw new ApprovalException( 'It\'s a conditional approval so please add some comments.');
        }

        // First let's fetch the current step in the approval process.
        List<ProcessInstanceWorkItem> items = [SELECT Id 
            FROM ProcessInstanceWorkItem
            WHERE ProcessInstance.Status = 'Pending' AND ProcessInstance.TargetObjectId = :sc.getId()
            LIMIT 1];
        if(items.isEmpty()){
            throw new ApprovalException('Looks like there\'s nothing to approve? Please go back to Opportunity or refresh the page.'); // somebody else has already approved?
        }

        Approval.ProcessWorkitemRequest req = new Approval.ProcessWorkitemRequest();
        req.setAction(validActions.get(opp.Approval_Status__c)); // use our mapping to figure out how to translate the action
        req.setComments(String.isBlank(approvalComments) ? opp.Approval_Status__c : approvalComments.abbreviate(4000));
        req.setWorkitemId(items[0].Id);
        Approval.ProcessResult result =  Approval.process(req);
    }

    public class ApprovalException extends Exception{}
}

<apex:page standardController="Opportunity" extensions="OpportunityApprovalCtrl" title="Custom Opportunity Approval">
    <apex:sectionHeader title="{!$ObjectType.Opportunity.Label}: {!Opportunity.Name}" />
    <apex:form >
        <apex:pageMessages id="messages"/>
        <apex:pageBlock title="Approve / Reject Approval Request">

            <apex:pageBlockSection columns="1">
                <apex:outputField value="{!Opportunity.Name}" />
                <apex:outputField value="{!Opportunity.OwnerId}" />
                <apex:outputField value="{!Opportunity.AccountId}" />
                <apex:outputField value="{!Opportunity.StageName}" />
                <apex:outputField value="{!Opportunity.Amount}" />
                <apex:inputField value="{!Opportunity.Approval_Status__c}" required="true"/>
                <!-- This is for conditional approval (to store on Opportunity). -->
                <apex:inputField value="{!Opportunity.Approval_Comments__c}"/>  

                <!-- This is for standard comments field (to store in approval history). -->
                <apex:inputTextArea label="Comments" value="{!approvalComments}" cols="100" rows="5" />
            </apex:pageBlockSection>

            <apex:pageBlockButtons location="bottom">
                <apex:commandButton value="Save" action="{!save}" rerender="messages, steps" status="status"/>
                <apex:commandButton value="Cancel" action="{!Cancel}" immediate="true"/>
                <apex:actionStatus id="status" stopText="">
                    <apex:facet name="start">Requesting... <img src="/apexpages/devmode/img/saveStatus.gif" /></apex:facet>
                </apex:actionStatus>
            </apex:pageBlockButtons>
        </apex:pageBlock>
    </apex:form>
    <apex:relatedList id="steps" subject="{!Opportunity.Id}" list="ProcessSteps" />
</apex:page>