I see you changed the "Affects version" field. I took the liberty to correct that; that field is for what version of Weld contains thig bug or is affected by lack of the feature in this case.
Both, Weld 2 and 3 are affected here and it's good to list as specific version as possible which is the latest here.
Weld version 2.x is in a maintenance mode.
Only Weld 3.x is actively developed now, so that is where the patch needs to go.

As for how to contribute - it is a classic GitHub pull request process.
You fork and clone https://github.com/weld/core, then you work on the fix on your fork and base it against master branch. Once done you send a pull request against original repository,.
Please include this jira number (WELD-2582) in pull request name/commit message.
Let me know if it's still unclear.