This bit is confusing, so just to clear things up, each mesh body is it's own shape, and each mesh body will have its own weights. For e.g. Hourglass Petite is not going to fit eBody Curvy. So..
Get yourself a copy of each Dev Kit. Top branded ones like Maitreya, Slink, Belleza, Legacy etc will have an application form on their own websites for you to fill out. and some brands will have a kit you can pick up in their store
Get Avastar. Though rigging can be done with native blender tools, most dev kits will come prepared to be used in the likes of avastar. Avastar will also help in so many ways when it comes to rigging.
Make your base garment and then size it to shape of each mesh body, and then you will need to rig and paint it for each body.
Get a copy of each body to test your fitting. Demo bodies work just as well as full versions for this.
Get signed up for the Beta Grid so that you can upload your items for testing
If you can't get the Dev Kit you want most at this time its no big deal, build up a portfolio using other body types, then you can reapply later when you built up something they can see.
To get on the Beta Grid, if you do not already have permission, you will have to file a ticket with Linden Lab. And you will need to read and agree to the IP Terms on both Agni & Aditi accounts to be granted mesh upload ability. http://wiki.secondlife.com/wiki/Preview_Grid
This statement I quoted sounds rather flippant. Retopolgy isn't just about putting new mesh on existing. For starters you gotta have an existing model to retopo and that means making it usually, it can be used for cleaning up geometry, making good edgeflow and, importantly for SL, reducing the overall triangle count. Rigging in blender has quite the learning curve, and the curve isn't reduced by having Avastar. Familiarise yourself with the different bone types that SL uses, how to assign materials/faces etc in blender. Learn how to unwrap, bake AO's, bake normal maps from high poly to low. How to bake textures if thats the way you want to go. And some limitations that SL has in regards to mesh http://wiki.secondlife.com/wiki/Limits#Mesh
Personally, if i was starting again, from scratch, I'd do the same as i did first time round. Start without avastar and using the default SL collada which you can get from the wiki, or better yet get the workbench files free from the avastar website, and practise. Practise modelling the mesh, practise basic rigging to just deform bones. then move up to get avastar and with the likes of eBody or Tonic which are easy kits to get in both stores, then practise with those dev kits, and adjusting sizes.
I know i've missed stuff out, my brain kinda has things on auto pilot that i cant remember anymore it just "does it" lol. And whether we use blender & avastar or maya & mayastar or whether we stick to native tools, there's a ton of different workflows out there so you'll probably get different, and possibly conflicting answers.
Don't expect to find many videos that takes you through from step 1 to step finished, if you are looking for tutorial guides you will likely find them broken down into different segments, there may be a video for making the original garment, then another for unwrapping, another for the baking, another for resizing, another for rigging and so on and so forth. The trick to the search is really to know what stage you are searching for, and though some videos may appear to be old, for the most part they are still relevant in some way, but the programme you use now may differ a bit to the one used in a video made in 2013 lol