Okie doke lovelies, we're all ready to get these gussets attached now and don't worry, you've already done the hard bit!
Firstly, if you haven't already marked the centres in the seam allowances of the raw edges, do it now!
Lay the outer base gusset right sides up with one short edge facing you. Lay the lining gusset on top right side down. Line up the short edges and if you need to trim the short edge of the zipped gusset slightly then do that now. Don't trim the zip!
 Pull the lining zip gusset back and pin into place. I like to do this with a double fold as shown above. Fold back then fold back again and pin into place.

Stitch across each half of the zipped gusset, either side of the zip. You can sew over the zip tape as shown above, but don't sew over the lining piece. You will close the gap left over the zip after. Remember to backstitch at each start and end of sewing.

Repeat for the other short end of the outer base gusset and the zipped gusset.

Next lay the lining base gusset down face up and lay the zipped gusset on top as before, this time it has the outer base gusset already attached.

Pin the outer pieces together to pull them back away from the lining and stitch as before. You will not be able to get as close to the zip tape as before because of the bulk of the outer.
 Now you should have something similar to the picture below. The outer gusset is only attached to the outer zipped gusset. The lining gusset is only attached to the lining zipped gusset.
 Turn this over on itself so that the wrong sides are facing and the lining is inside the outer.
 Flatten the seams where the two gussets meet and then top stitch on the base gusset, just the width of the zip.
 Be sure to only stitch as wide as the zip. I stitched forwards, then backwards and forwards again to secure and tied off my ends.
 This little row of stitching will be hidden by the triangular (or D) rings. You should not be able to see your side tabs like the picture below unless you have left off the side elasticated pockets.
This way of attaching the gussets together is the key to being able to use a turning gap on a gusseted bag and you're really on the way to a great looking bag now! The hard bit is definitely over, it's all fun, fun, fun from here!