Once you created an asset and you want to pay dividends to your share holders, just switch to the asset manager account (in portfolio click on the asset Ticker) and then visit the asset view page and you see an administrative console with options Raise Motion & Pay dividends.
To pay dividends, follow the link and enter the total amount of btc you want to distribute to share holders.
Only shares that left the asset manager's account are entitled for dividends and vote in motions. To find out how many shares will receive dividends, see how many shares you issued and how many of those were sold or transferred out from the asset manager's account.
If you have unsold shares or shares you bought back from the secondary market, those shares will not receive dividends nor have a right to vote in motions.
Unless you need to pay a precise amount per share you can let GLBSE take care of it and just enter the total amount you want to distribute.
This guide is for GLBSE 2.0 with only 1 option to pay available. If this will change in future, I'll try to catch up on that development.