Specifically to Windows SharePoint Service 3.0, creating a custom form is rather easy. It took me a while to get this routine down and now I can create a custom form entry screen in minutes. When I say customizing a form, I'm speaking specifically to changing the layout of the EditForm, NewForm and DisplayForm pages for a list. You can customize these pages so you can have more columns than provided, make the form match the original paper form or remove certain fields from the NewForm but you want on the EditForm.

First, Create the list! Yeah, an easy one. I suggest ordering the columns in the List Settings area of SharePoint before trying to customize the forms. Also, the larger the list, the more painful this may be. We'll be using SharePoint Designer which appears to be a hog of an app on certain computers. From what I can tell, every change you make, like dragging a field to another cell in the table, SD will refresh the page and requery SQL for all of the field names and properties. This process can take a long time if you have 30 or more fields. I have a list with about 60 and it's a good 10 seconds before I can edit another item on the page.

Second, Open the WSS site in SD. Go to the list and the page you want to edit, for example NewForm.aspx. Right click on NewForm.aspx and select Copy. Then right click on the List Name and click Paste. This will create a backup copy of this form in the even you blow it up, which I have done so many times. Doubleclick and open the original file, NewForm.aspx.

Third
A. You will see a three field web part in the middle of the page. This is the default webpart that displays whatever fields in whatever order they are defined in the list settings.

B. Switch to Split view in SD so that you can view code and the designer. Click on this web part and the code above will then highlight the code that makes up the web part. Look for tags IsVisible and change it from true to false. This will hide the default form.

C. Click on the web part in the design view again then press your right arrow. This will move the cursor to right after the web part but still within the web part zone. Go to the Insert menu, select SharePoint Controls then Custom List form. A small dialog window will appear, select the list name from the first drop down, then select the content type from the second drop down. Finally select which form you want to insert, New, Edit or Display. Click OK. The new form will be displayed under the default form, and you will then see all of the fields in order.

Finally
Now customize your form! Move fields around, delete rows, change formatting, whatever you need to do. Click save and when you go to access the form through the SharePoint site you should see your new updates.

A few notes:
I have found that it is important for the default form to remain, step Third B is important to complete. If you delete it, your new custom form will work but the list may not default to your form for the sake of creating a new item, or editing. If you go to the Properties of the list in SD, then go to the Supporting Files tab, you will see what the three files the list relys on for displaying, editing and creating. If you delete the default form in the NewForm.aspx, the New Form item field in the Supporting Files property may be empty and is a pain to get back in there until you put the list form back into your page.

When you change a form to a custom view, you lose the history list for multiline textboxes. To get it back, put the following line of code where you want the history to appear:  < SharePoint:AppendOnlyHistory runat="server" FieldName="Comment" ControlMode="Display">


Thanks for reading, I hope this helps!!