A sissy girl should have tan lines that perfectly show they wear a string bikini. If you have a garden or terrace, lock (tie) him down to a sun bathing chair wearing a full bikini and leave him there for an hour so he develops some nice tan lines. Alternatively, bring your sissy as a surprise to a tanning salon wearing a bikini (top and bottom) or a bra and panties, and escort him into the tanning cabin. Handcuff his hands over his head to the bar above his head inside the tanning cabin and give him a nice long tanning session.... you can repeat this several days in a row. Then offer him a massage afterwards, by a very cute woman.
This is an absolutely delicious way to humiliate your sissy and to keep constantly reminded that he's to be treated as a sissy girl until the tan lines disappear, which may take several weeks. It's a fantastic punishment if you catch him without a bra or in some way refusing to wear the feminine clothes you require him to. Every time he looks in the mirror he will be reminded of his predicament. He will also have to avoid being seen naked in the gym and other situations.
You should sweetly, softly and firmly encourage him to get a nice tan, explaining to him how sexy he will look . Tell him you will only allow him to tan in a full bikini or bra and panties. You can also explain to him that this is the punishment for not wearing a bra regularly under his clothes, and that if he doesn't feel comfortable wearing one, then he will have to have visible tan lines. Then you can turn it up a notch once he has the tan lines by threatening to make him change his shirt in front of another woman or girlfriend. You could even follow through with the threat by having him try on shirts in front of a sales girl at a shop.
Make sure he's wearing a bikini (top and bottom) or a complete bra and panties every time he tans, and make sure the garments are in the exact same place each time, so the tan lines become more evident. You can also take the opportunity to write "Sissy Girl" in strong sun block cream on his back or stomach while he tans so that he has an additional tanned reminder.
You should do this as a surprise when he least expects it, or as a punishment when he's caught without feminine lingerie on.
Bring him to a tanning salon and accompany him to the cabin. If you have to explain to the attendant, It shouldn't be a problem. You can have the tanning salon woman either know or not know what's going on - you decide. If you have her know, you may want to have her come in at the end to unlock his hands. You may also want to enlist a girlfriend to help you with this delicious idea.
This is only exciting if you Force it on him. Don't let him get away with going into the tanning salon himself without you, he will not feel forced, and it will not have the same effect. Make sure you lock his hands to the tanning cabin bar, so there is no way he can move or remove what he is wearing. Make sure you re-enforce a tan a couple of times, and make sure you have hi in chastity or have some other method of making him go to the tanning salon. Don't get afraid of him having some tan lines, they will go away after a few weeks, no problem.