Provisional Cast On
With this type of cast on you can keep your cast on stitches live to easily be picked up again at a later time. This technique gives you a seamless transition. It's also perfect for knits that need to be reversible - so it looks great from both sides.
I'll show you here how to make this cast on with leftover yarn and your knitting needles.
Casting On
Place the leftover yarn (brown) over your left thumb and your main working yarn (green) over your left index finger, hold the ends of the yarn with your right hand.
Move your thumb upwards so that a loop is formed - as you would for a German Twisted Cast On.
Go through the back loop over your thumb from left to right ...
... and yarn over with your main working yarn.
= 1 stitch cast on.
Repeat this until your necessary number of stitches is reached.
Connecting Provisional & Live Stitches
Fold your knitted piece so that both wrong sides are facing each other and the provisional cast on is behind the active sts.
Insert your left needle into the first st between the leftover yarn (grey).
Work the provisional and active sts together in the st pattern.
Continue to insert your needle into the adjacent provisional st, lift it onto the left needle and work it together with the active st.
Finally, when all provisional sts have been worked, remove the remaining leftover yarn from the cast on on the wrong side.