I've been experiencing the joys of upgrading to 20mm rubbers on my RA railgun recently - so I'll share my learnings (search for Rob Allen or 20mm rubbers and you'll prob. find more info.). I believe Omer intructions show loading against your belt buckle -- never tried that, bit close to "the old home town" for comfort perhaps.
Main thing: make sure you've got a really good loading pad on your chest/sternum/breastbone. I
supplement the one on my Elios wetsuit with the sole of fancy flip-flop (thicker & harder than those cheap rubber flip flops). Long term, it might be unnecessary but when starting out, it certainly helps - a lot.
Load using normal technique -- but do it carefully:
Grab one side of your rubber & place the handle of your speargun firmly on your loading pad. (You might find this works best if you point the gun down, optionally trapping the end of the barrel with your fins, it seems to keep help -- I got this from the RA/Len Jones* diagrams that are widely published. I started off like that & it helped, although it no longer seems necessary).
Grab the other side of your rubber. Now start to
brace yourself by tensing your arms, chest, abs and shoulder muscle. Now pull both arms into your chest in one firm, controlled pull (focus as if lifting a heavy weight), until you can slip the wishbone into the notch.
Good luck
*The Len Jones spearfishing booklet shows 2 loading techniques -- the one on an extra loose page appears to be intended for those loading very long railguns. The techniques are similar though.