Reddit mentions of Fly Rod Building Made Easy: A Complete Step-by-Step Guide to Making a High-Quality Fly Rod on a Budget

Rod handles are actually pretty easy to install, provided you have a 4 piece rod with no guides on the tip end of the piece.

The typical process to install a grip would be to epoxy on the reel seat, then slide a pre-formed cork grip (available from REC) down to mate with it. You build up tape arbors underneath the grip with masking tape to get a nice snug fit, and you epoxy it in place. Then you simply add a winding check and typically do a cosmetic thread wrap ahead of that.

Art Scheck's book "Fly Rod Building Made Easy" is the definitive guide to this kind of process, and is a great read as well. https://www.amazon.com/Building-Made-Easy-Step-Step-ebook/dp/B0058JNLWC

You can get the hardware here:

https://reelseats.com/shop/index.php?main_page=index&cPath=26_144_146&zenid=cc6605ba335d32f86b5cffce9ec01f8b

This may not be as cost-effective as buying a new budget rod (depending on what kind of rod you're talking about here), but it's a great set of skills to know and you'll have something you can take pride in when you're done. All you need to do is remove the original cork carefully with whatever seems best to you (I'd use a box cutter to carve and break the cork off then I'd carefully sand the cork core and original tape arbor down to close to the original blank with 60 grit sandpaper, leaving a little bit of the original tape arbor for insurance since you don't want to scratch the graphite and weaken it). Then you build up from there.