MultiMC - alternative Minecraft launcher
MultiMC is a free, open source launcher for Minecraft. It allows you to have multiple, cleanly separated instances of Minecraft (each with their own mods, texture packs, saves, etc) and helps you manage them and their associated options with a simple and powerful interface. The launcher is very good for Linux users or those who don't like the Twitch launcher.
Download here: https://multimc.org/#Download
After installing, you need to change memory settings and add profile
How to add a modpack
How to find or upload crash logs:
With each modpack launch a Console windows opens where you can see log being generated. If modpack crashes, the reason for it can be found there. Also you can upload the log with 1 mouse click.