Before spooling braided or superlines onto the spool you must use monofilament lines as backing.
The exception will be when using one of the newer reel models e.g. Abu Garcia Soron SX‘s braid-ready spool that eliminates the need for mono backing.
The use of monofilament line as backing is to prevent slipping of the braided lines when fighting fish which renders your reel drag redundant. Braided line also slips when wet. I know this because I have tried it.
The following photo is meant as a visual reference of how the spooled line will look like. In this case the yellow line will represent the mono backing.
Connect the mono to the reel spool using an Arbor Knot. How much line is needed is determined by your spool capacity and the length of braid that you intend to have on the spool.
I frequently join the braid with the mono using an Albright knot. Sometimes tying an FG knot if using thicker diameter mono or when I’m not too lazy! I know of friends who prefer a Bimimi. It’s all down to preference as no well tied knots have failed thus far.