Assuming you don't think it worth engaging lawyers, you need to simply specify three essential things, and invoice in the usual way. You grant them a licence to use:
(enter name of recording), version (as in how you describe it) composer (as in you), recorded by (put your name here too, plus anyone else who could have had input), duration, any registration details, if you lodged it with any official licencing body - just in case they spot the use and think it unauthorised, and then where it can be used, period, and medium. If the period is effectively 'forever' then the correct word is in perpetuity. The media itself needs identifying - so nowadays there is a big difference with streaming and downloading being treated differently.
So taking one of ours, as an example,
You are granted a licence to use the track detailed below.
Title-------- Version----------------Duration--Composer------------Engineer------ISRC------------ Period-------Region--------Media
Dissolution-Instrumental version--3:04-------Paul Robert Johnson--Paul Johnson-QZFZ52140495-In Perpetuity-Any Territory-Any medium
However, I don't do in perpetuity (as in forever) any longer - I always pick a time period. 5 Years is a long time, but it does mean that after 5 years, you can market it yourself again if you want. Usually the product is dead by then, but occasionally it won't be and you can negotiate a repeat. For big companies, expect them to provide you with terms and conditions - sort of a reverse agreement which will be a proper legal document, which you can agree with or not, but they'll be the same basic headings but reversed. For small concerns it's also sensible to ask a question, or give them an option, which will generate a response - even in an email. Perhaps remove Any territory and ask them if they'd prefer to licence it specifically for their own country, or licence it for worldwide? There's no change in the price. They will respond then and you enter 'Any territory' and send it back. It would, if pushed be useful if things went sour - you'd have an evidence trail showing what they asked for and the details.
These details are enough to keep people happy, and loose enough to not tie your hands behind you.
If there was a lot of money at stake, real legal advice should be taken, but this has worked well enough for me, and is less 'threatening' for amateurs and small companies to agree to. What it is - how long can it be used - where can it be used and of course you can add money details too.
The only money issue is simply to take points, or a fee, or both? None of my music has ever earned enough for me to be grumpy about taking the set fee. 100% of something small being better than a percentage of nothing.