How to create a button in flutter that always sticks to the bottom of the page?

You can copy paste run full code below
You can use bottomSheet attribute of Scaffold

return Scaffold(
      ...
      bottomSheet: Container(
        width: MediaQuery.of(context).size.width,
        child: RaisedButton(
          child: Text('PROCEED'),
          onPressed: () {},
        ),
      ),
    );

working demo

full code

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: BottomSheetTestPage(),
    );
  }
}

class BottomSheetTestPage extends StatefulWidget {
  @override
  _BottomSheetPageState createState() => _BottomSheetPageState();
}

class _BottomSheetPageState extends State<BottomSheetTestPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('bottom sheet'),
      ),
      body: ListView.builder(
        itemBuilder: (BuildContext context, int index) {
          return ListTile(
            title: Text(index.toString()),
            subtitle: Text("${index}"),
          );
        },
        itemCount: 300,
      ),
      bottomSheet: Container(
        width: MediaQuery.of(context).size.width,
        child: RaisedButton(
          child: Text('PROCEED'),
          onPressed: () {},
        ),
      ),
    );
  }
}

You will solve using Expanded widget.

       Expanded(
          child: Align(
            alignment: FractionalOffset.bottomCenter,
            child: MaterialButton(
              onPressed: () => {},
              child: Text(’Proceed'),
            ),
          ),
        ),

You can expand the first part of the layout

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Column(
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: [
        Expanded(child: Text('FirstPart')),
        InkWell(
          onTap: () => print('pushed'),
          child: Container(
            width: 100,
            height: 60,
            alignment: Alignment.center,
            color: Colors.black26,
            child: Text('PROCEED'),
          ),
        )
      ],
    );
  }
}

You can use stack like this

Stack(
        children: <Widget>[
          //your other widgets here
          Align(
            alignment: Alignment.bottomCenter,
            child: yourButton,
          )
        ],
      ),

a simple way to do this is using a Scaffold since the scaffold has a bottomNavigationBar attribute that takes a widget then you can simply pass to it a button like this

 Widget build(BuildContext context) {
    return Scaffold(
      // will always be at the bottom
      bottomNavigationBar: FlatButton(onPressed: (){},child: Text("Proceed"),),
      body: Text("your body goes here"),
    );
}