Typically the boat is supported on each rail, just aft of the balance point, under the centerboard trunk on the centerline, also just aft of the balance point, and then near the bow, on the centerline.
All you are trying to do when you tie the boat down is keep it on the trailer. To do this, you need to stop it lifting off, and you need to stop it sliding backwards. A strap holding the boat down, at or in front of the dolly gunwale supports usually suffices to keep the boat down. I also tie the bow down and forward to the trailer, by fastening a line to either the mast gate or the ram attachment point on the deck, and leading it forward to the bow. With carpet or foam carefully added to prevent the lines wearing into the rail, I tie the line to the dolly handles. This keeps the bow of the boat down, and also stops the boat sliding aft on the trailer.
Try to eliminate movement, as this leads to wear. Pad straps and tie down lines, so they do not wear into the boat.
I leave sails and other loose gear in the boat, though I usually tie sailbags in, and leave the rudder in a rudder bag, lying on the floor
Anything in the boat that could move or bang, should be tied off and/or padded. This incldues the trapeze hooks (usually left on the shockcord), and the boom and spinnaker pole.
The mast is usally supported at or near the stern of the boat on a lighting board, and on a forward support near the front of the trailer. Lighting boards can bounce off the transom gudgeons (if that is how you hang yours), and masts can bounce out of both supports. Tie the lighting board down, and tie the mast down as well.
Always trail with a bottom cover.
Mast preparation. I normally take forestay and trapeze wires off the mast, and coil the shrouds up to the spreaders. I sky the halyards, by pulling the tails until the other end is jammed against the fitting in the mast it turns around. This minimizes wear on the mast. Tie the coiled shrouds to the spreaders.
Many people use an old sail bag over the bottom of the mast, to keep the coiled halyards tails and various shackles etc. with the mast, instead of falling off onto the highway.
Do not trail with a top cover unless you put it inside the bottom cover. You may wish to reinforce the top cover where it touches the mainsheet cleat - they tend to wear there.
Another top cover option is to tie it up to the mast, so it does not chafe against the mainsheet cleat and other gear in the boat.