I suspect a lot of it is read off the Cisco Data sheets on the fly.
That's right, you'll spend a lot of time reading these, so worth bookmarking the common ones in your browser.
Cisco has a configurator within their CCW section. These is very useful, because it lists out all the possible part numbers associated with each product. You still need to do your research, but it's better than nothing. For example, it won't tell you that you only need one VPN license for a HA pair of ASAs. It won't tell you that you'll need a single 1m stack cable and for a large stack of 3750 switches.
There is so much Cisco product information that you can never learn it all. I keep my notes updated and list out key points for the common products. MS OneNote might work well, although I still keep everything in a huge Word doc. If I learn anything new and important, then it gets added. Can be a pain, but it's a great reference. The only downside is that old info must be cross-checked, because things change. For example, features can sometimes drop down the IOS feature sets as time goes by.
Take the most commonly sold products by your partner and build up some good notes on these first. Perhaps create some tables identifying key features, or use columns with tick boxes - i.e. Dual PSU?, stackable?, etc.
Make sure you understand how the part numbers are constructed for your key products. For example, the part number for a common fixed-chassis switch like the 2960 or 3750 will tell you the number of switchports, type of switchports (RJ45/SPF), PoE (none/low/half/full), type of uplinks (RJ45/SFP/both), speed of uplinks (1/10GbE), type of IOS feature set etc etc
Learn the key differences between the IOS feature sets for the most common products. This is important. You don't want any nasty post-sale surprises where the device won't perform a feature requested by the customer. It can be expensive.
Cisco CCDA and CCDP exams will help with any design work.
Cisco Live! 365 videos are good, so worth signing up www.ciscolive365.com
Cisco Design Zone can be useful, but will depend how low-level your pre-sales role will be. Some roles are quite high level.
Learn Visio (or similar) and create a base template to work from - i.e. get all your common icons and components ready, plus your subject box (i.e. title, customer, author, date, internal reference number etc)
Template a nice long list of common caveats to add at the end of your proposal - i.e. customer to schedule network outage window, customer to supply fibre patch leads, customer to make available necessary rack space etc etc etc. Just take out the ones you don't need each time.
Ask colleagues to vet your proposals whilst you're still learning.
There's probably more, but that should help get you started.
