It's never too late to have a new favourite method for doing something!
I always thought invisible zips were my super power. However getting them to join smoothly with the seam at the bottom is always a challenge.
This method stitches the seam first.
The top of the zip pull needs to sit about 2-3mm below the stitching line at the top.
The opening for the zip needs to be long enough to get the garment on and off, but always at least 3cm shorter than the zip teeth. Therefore the zip needs to be longer than the opening.
Mark a line across the zip tape about 2cm above the bottom stop.
With the top of the zip pull just below the top stitching line use the zip to work out where the zip opening should end. The line you marked is the length of the opening in the seam.
Stitch the seam below this point, reinforcing the stitching at the end of the opening. Then use your longest stitch to close the opening either 10 or15mm from the edge depending on your seam allowance.
Press the seam open and draw a line across the seam allowances where the bottom of the opening will be.
With the zip pull down against the wrong side of the garment match the lines you've drawn on the zip tape and the seam.
This pic was takem later, so please ignore the machine stitches.
Carefully match the centre of the zip to the seam
Pin the zip tape to both seam allowances, but not the main fabric. If you prefer you can use Wonder Tape. This is a double sided tape that eventually washes away.
8.Machine through the zip tape and the seam allowance. You might need to move the needle position so you can get past the zip pull. Stop level with the line you drew on the zip tape, it makes things easier later.
After stitching both sides of the zip you remove the basting from the zip opening. I prefer to pull this stitching out from one side, if you use an unpicker the cut threads can get caught in the zip.
Open the zip right down to the bottom stop.
Change to your invisible zip foot.
I like to use a pin, but you could tack if you want. Roll the zip teeth open and carefully pin very close to the teeth so the pin matches with the stitching at the top of the seam on the other side.
Use the zip foot stitch from the bottom of the zip towards the top. Note which side of the foot the teeth are under.
Start stitching exactly at the line you marked on the zip tape. You can use a lock stitch or fasten off securely later. Use a finger to roll the teeth open and stitch really close to the teeth.
Repeat with the other side of the zip, starting at the bottom of the zip stitching to the top.
To fasten the zip for the first time you need to push the zip pull between the zip tape and the fabric. I find it best to have the garment with the wrong side on top.
Grab the zip pull in one hand and the bottom of the zip tape with the other. Pull to close the zip.