Welcome, ksimpson.

Glad to see you here in the QuickBooks Community. Let me help you get across this challenge so you can use your own estimate template to QuickBooks Online.

Since your estimate template is already in a Microsoft Office Word file format, make sure to mark the text that you want QuickBooks to map data with opening and closing angle brackets. Never mark the data that don’t change (company name, address, and contact information).
QuickBooks support the following font styles:

• Arial Unicode MS.
• Courier, Courier Bold, Courier Oblique, Courier Bold-Oblique.
• Helvetica, Helvetica Bold, Helvetica Oblique, Helvetica Bold-Oblique
• Times Roman, Times-Bold, Times-Italic, Times Bold-Italic
• Symbol
• Zapf Dingbats

Also, be sure to use Microsoft Word tables to contain the contents and create the table layouts with simple rows and columns. Merge/split cells and tables placed inside of tables can cause unexpected problems.
Avoid these following elements as they can cause issues with the import:

• Audio
• ClipArt, Word Art.
• Animated GIFS.
• Embedded files.
• Adding or updating hidden links or merging fields.
• Files over 2MB.
• Non .docx file types or older MS Word documents renamed as .docx files.

To correctly import your own estimate template, please follow these steps:
1.    Click the Gear icon, and select Custom Form Styles.
2.    Choose the New Style drop-down and select Import Style.
3.    Under the Select a form type, choose Estimate.
4.    Download a sample estimate for your reference, and then click the Browse button to upload your own file.
5.    Select Next to upload your document.

Now, you need to map marked areas in your document to QuickBooks data fields.  Review each field to make sure the mapped fields from your template correspond to the correct fields in QuickBooks Online. Check if the numbering and mapping of fields are done correctly.

Once everything is linked correctly, select Next to see a preview of your invoice. Choose Customize and select the saved customer template.

And that’s it! Your custom form is ready to use. Please let me know if you have other questions. I'd be glad to help you further.