You can get to several of the Sill routes from the south fork of Big Pine Creek, but I don’t like that approach. I think that most climbers use the north fork of Big Pine Creek, cross the glacier, and climb the North Couloir route. I wouldn’t climb Sill from the west unless I was already on that side to do something else.
Crossing the glacier requires good basic mountaineering skills and equipment. The glacier gets steep near Glacier Notch. The rock above the glacier is rotten. The North Couloir is steep enough to require your full attention, but it’s not very difficult. It’s all rock from the top of the couloir to the summit. With perfect route finding, it’s only third class. It’s more work than Humphreys, but less technical.
The way you improve your skills is to get out there and climb stuff, slowly increasing the difficulty of what you do on your own. Most of us are mostly self-taught. If you never climb with a guide, you may miss out on learning from the best. If you always climb with a guide, you probably never learn to organize a trip and make your own good decisions. I would occasionally hire a guide, if you can afford it. You may also find climbing classes hosted by clubs, schools, or guide services.
As you know, I did Thunderbolt to Sill in 1997 and Sill to Thunderbolt in 2000. The key was practicing these entire traverses in sections before putting everything together. I would just keep climbing as much as you can, especially the 14ers. You’ll know when it’s time for the traverse.