Undo the mounts, disconnect all the controls, water system and exhaust, undo the bolts connecting to the gearbox and slide forward. You may have to provide something such as some blocks of wood at the forward end to support it. Depending on the access to the back you may find it easier to rig up a block and tackle to a beam across the companionway to lift the engine right up and forward into the boat so that you can remove the saildrive which comes out upwards.
The first thing that helped when I did mine was that over the few years prior to doing the saildrive diaphragm was that I'd replaced most of the engine hoses and made each of them a few inches longer - This meant for an easy engine move fowards with no hoses to disconnect on the day.
As to support - You know those cheap plastic chopping boards? .. They're really slippery when you put a few on top of each other. I used a few in the engine bed and a few under the engine and to get the precise height just put a few wedges (like you find around the yard chocking up boats) point to point between the chopping boards. By pushing the wedges inwards towards each other I could jam the top chopping board up enough to fully support the engine. The engine easily slid fowards the few inches necessary with no drop/droop at all supported by the wedged chopping boards.
As to supporting the saildrive - Not sure if yours will then only have a single aft support - but saildrives like the 120s have deep set holes each side on their forward top end that bolts (on wooden supports perhaps) can be slid into.
All supported well, then engine should slide fowards and nothing else will move
When i did mine VP2001 i didn't need to move it far at all. I didn't remove any cables or pipes and just undid the mounts as others have said. Undo the drive first and stick something under the leg to support it, leave the bolts just in, two bolt holes at the side can be used with a rod or bolt to support the top, then slide the engine away. The leg then lifts away (backwards) from the engine and out it comes.
You may need to a little work on the ring that holds the diaphragm in but thats about it. PBO did a guide which can be found online and its very helpful.