I highly, highly, highly recommend Megan Nicolay's Generation T: 108 Ways to Transform a T-Shirt. It's a great resource. I have at least a dozen post-it notes sticking out of mine.
The book starts off with instruction of the various hand stitches that are used throughout the book should you opt to use hand stitching over a serger and/or sewing machine.
From there the chapters are organized by projects with the same themed end result. All use t-shirts as the starting point regardless of the outcome. There is a chapter on modifying tees that still resemble tees as well as chapters on punk, halters/tube tops, skirts/dresses, accessories (from scarves to handbags), a chapter full of smaller projects for those who don't like throwing away scrap pieces and the grand finale is a prom/wedding dress.
The project instructions are usually pretty clear but aren't exact measurements because the book wasn't made just for you.

There's a lot of "now but the _____ on, pin it wear it feels most comfortable and measure from here to here" type of instruction. But, you're learning custom tailoring at it's simplest form and know that it will fit just the way you want it. Most of the garment projects have recommendations on how to achieve a similar look but with limited or no sewing required.
Here's a link to one of the tutorials that she did with Threadbanger. The instruction starts at about the two minute mark. Prior to that is an interview with her:
How-to Make a Halter Top with Megan Nicolay, Threadbanger | TB Projects - DIY Fashion and Style
Have fun!