I work with a double thread on quite a few of my projects, and I often have to add a new thread. Here's how I do it: Work with the old thread until there is about 4" left. Stop, thread a new needle, and start this new thread about 4 or 5 rows back from where the old thread waits. Make a few overhand knots along one of the sides, not through the middle beads, and exit the bead where the old thread is. Tie a square knot and continue beading with the new thread.
When the beading has progressed for about an inch or so, pick up the old thread and work it up along the side making several overhand knots. Cut off the needle and tie the ends in a square knot before clipping or burning the thread close to the beads.
The bead that I'm using in this base has 9 beads to the inch along the side. If you want to know how many beads you'll need for the base, multiply this number times three. In this case, twenty-seven beads to the inch or 182 for a 6-3/4" base.
I use the new 8mm heavy magnetic clasps on a lot of these bracelets. (If you can't find them at your local bead store, Cindy Tipton, the Beadlady, has them available on line.) Stop when you have a "Straight" row coming up, and the magnet will fit right in the gap. Take the thread through the first bead, string on the magnet, take it through the last bead. Now come through the bead right below and start working overhand knots along the thread between the bead and the magnet. Go through the magnet and work overhand knots on the thread between this side of the magnet and the end bead. (It's like sewing on a hook with buttonhole stitches.) Work the thread along the side and finish it by clipping the thread leaving enough to tie a square knot, before clipping it close. I add the other half of the magnet the same way on the other side by threading the needle with the beginning thread, adding one bead to the side to get to the "Straight" row.
Once you've made a few of these you'll be amazed at how quickly you can create a base. Next week I'll be building a bracelet on this base. It's going to be "Miss Beddie's Pond" with a new flower made from petal beads.
Here's another bracelet with the flowers made from leaves. I titled this one "The Secret Garden," because you have to look really hard to find the flowers.
I'm off tomorrow for a weekend at the Shreveport Louisiana Bead Retreat. I'm teaching a new piece, the Rose Vine Bracelet. I'll also be teaching this bracelet at Beadoholique Too in Houston during April.